[wasm] Add support for a random test case orderer, for xunit tests

This is enabled by default for wasm with
`$(XUnitUseRandomizedTestOrderer)=true`.

When the library tests run, they print two messages like:

```
  info: Using random seed for test cases: 700149826
  info: Using random seed for collections: 700149826
  info: Starting:    System.Collections.Immutable.Tests.dll
```

These seeds are picked randomly every time the tests are run. To run the
tests with a specific seed, use environment variable
`XUNIT_RANDOM_ORDER_SEED`.

When running with tests, this can be used as:

    `WasmXHarnessMonoArgs="--setenv=XUNIT_RANDOM_ORDER_SEED=<seed>`

eng/testing/tests.mobile.targets

@@ -51,6 +51,10 @@
     <AdditionalXHarnessArguments Condition="'$(XUnitClassName)' != ''">$(AdditionalXHarnessArguments) -- -c=$(XUnitClassName)</AdditionalXHarnessArguments>
   </PropertyGroup>
 
+  <ItemGroup Condition="'$(XUnitUseRandomizedTestOrderer)' == 'true'">
+    <Compile Include="$(RepoRoot)src\libraries\Common\tests\Tests\RandomizedTestOrderAssemblyInfo.cs" />
+  </ItemGroup>
+
   <UsingTask Condition="'$(RunAOTCompilation)' == 'true'" TaskName="MonoAOTCompiler" AssemblyFile="$(MonoAOTCompilerTasksAssemblyPath)" />
   <Import Condition="'$(RunAOTCompilation)' == 'true'" Project="$(MonoAOTCompilerDir)MonoAOTCompiler.props" />
 

eng/testing/tests.wasm.targets

@@ -15,6 +15,7 @@
     <_ShellCommandSeparator Condition="'$(OS)' != 'Windows_NT'">&amp;&amp;</_ShellCommandSeparator>
     <WasmNativeStrip>false</WasmNativeStrip>
     <_WasmMainJSFileName Condition="'$(WasmMainJSPath)' != ''">$([System.IO.Path]::GetFileName('$(WasmMainJSPath)'))</_WasmMainJSFileName>
+    <XUnitUseRandomizedTestOrderer>true</XUnitUseRandomizedTestOrderer>
   </PropertyGroup>
 
   <PropertyGroup>

src/libraries/Common/tests/TestUtilities/RandomTestCaseOrderer.cs

@@ -0,0 +1,77 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Threading;
+using Xunit.Abstractions;
+using Xunit.Sdk;
+
+#nullable enable
+
+namespace TestUtilities;
+
+// Based on https://github.com/xunit/xunit/blob/v2/src/xunit.execution/Sdk/DefaultTestCaseOrderer.cs
+
+public class RandomTestCaseOrderer : ITestCaseOrderer
+{
+    public const string RandomSeedEnvironmentVariableName = "XUNIT_RANDOM_ORDER_SEED";
+
+    public static readonly Lazy<int> LazySeed = new (GetSeed, LazyThreadSafetyMode.ExecutionAndPublication);
+    private readonly IMessageSink _diagnosticMessageSink;
+
+    private static int GetSeed()
+    {
+        string? seedEnvVar = Environment.GetEnvironmentVariable(RandomSeedEnvironmentVariableName);
+        if (string.IsNullOrEmpty(seedEnvVar) || !int.TryParse(seedEnvVar, out int seed))
+        {
+            seed = new Random().Next();
+        }
+
+        return seed;
+    }
+
+    public RandomTestCaseOrderer(IMessageSink diagnosticMessageSink)
+    {
+        diagnosticMessageSink.OnMessage(new DiagnosticMessage($"Using random seed for test cases: {LazySeed.Value}"));
+        _diagnosticMessageSink = diagnosticMessageSink;
+    }
+
+    public IEnumerable<TTestCase> OrderTestCases<TTestCase>(IEnumerable<TTestCase> testCases) where TTestCase : ITestCase
+        => TryRandomize(testCases.ToList(), _diagnosticMessageSink, out List<TTestCase>? randomizedTests)
+                    ? randomizedTests
+                    : testCases;
+
+    public static bool TryRandomize<T>(List<T> tests, IMessageSink messageSink, [NotNullWhen(true)] out List<T>? randomizedTests)
+    {
+        randomizedTests = null;
+        try
+        {
+            randomizedTests = Randomize(tests.ToList());
+            return true;
+        }
+        catch (Exception ex)
+        {
+            messageSink.OnMessage(new DiagnosticMessage($"Failed to randomize test cases: {ex}"));
+            return false;
+        }
+
+        static List<T> Randomize(List<T> tests)
+        {
+            var result = new List<T>(tests.Count);
+
+            var randomizer = new Random(LazySeed.Value);
+
+            while (tests.Count > 0)
+            {
+                int next = randomizer.Next(tests.Count);
+                result.Add(tests[next]);
+                tests.RemoveAt(next);
+            }
+
+            return result;
+        }
+    }
+}

src/libraries/Common/tests/TestUtilities/RandomTestCollectionOrderer.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+using Xunit.Abstractions;
+using Xunit.Sdk;
+
+#nullable enable
+
+namespace TestUtilities;
+
+public class RandomTestCollectionOrderer : ITestCollectionOrderer
+{
+    private readonly IMessageSink _diagnosticMessageSink;
+
+    public RandomTestCollectionOrderer(IMessageSink diagnosticMessageSink)
+    {
+        diagnosticMessageSink.OnMessage(new DiagnosticMessage(
+                                                $"Using random seed for collections: {RandomTestCaseOrderer.LazySeed.Value}"));
+        _diagnosticMessageSink = diagnosticMessageSink;
+    }
+
+    public IEnumerable<ITestCollection> OrderTestCollections(IEnumerable<ITestCollection> testCollections)
+        => RandomTestCaseOrderer.TryRandomize(testCollections.ToList(), _diagnosticMessageSink, out List<ITestCollection>? randomizedTests)
+                    ? randomizedTests
+                    : testCollections;
+}

src/libraries/Common/tests/TestUtilities/TestUtilities.csproj

@@ -43,6 +43,9 @@
       variant from the Common folder and adding the missing members manually.
     -->
     <Compile Include="Interop\Interop.Libraries.cs" />
+
+    <Compile Include="RandomTestCaseOrderer.cs" />
+    <Compile Include="RandomTestCollectionOrderer.cs" />
   </ItemGroup>
   <!-- Windows imports -->
   <ItemGroup>

src/libraries/Common/tests/Tests/RandomizedTestOrderAssemblyInfo.cs

@@ -0,0 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+[assembly: TestCaseOrderer("TestUtilities.RandomTestCaseOrderer", "TestUtilities")]
+[assembly: TestCollectionOrderer("TestUtilities.RandomTestCollectionOrderer", "TestUtilities")]