Try to fix a time-sensitive test, enable some tests on Mono based on #…

…34582

src/libraries/System.IO.Pipes/tests/AssemblyInfo.cs

@@ -3,5 +3,4 @@
 
 using Xunit;
 
-[assembly: ActiveIssue("https://github.com/dotnet/runtime/issues/34582", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
 [assembly: SkipOnPlatform(TestPlatforms.Browser, "System.IO.Pipes is not supported on Browser")]

src/libraries/System.IO/tests/StreamReader/StreamReaderTests.cs

@@ -141,10 +141,24 @@ public async Task ReadToEndAsync_WithCancellation()
             File.WriteAllLines(path, Enumerable.Repeat("A very large file used for testing StreamReader cancellation. 0123456789012345678901234567890123456789.", 1_000_000));
 
             using StreamReader reader = File.OpenText(path);
-            using CancellationTokenSource cts = new (TimeSpan.FromMilliseconds(50));
+            using CancellationTokenSource cts = new ();
             var token = cts.Token;
 
-            var ex = await Assert.ThrowsAnyAsync<OperationCanceledException>(async () => await reader.ReadToEndAsync(token));
+            var ex = await Assert.ThrowsAnyAsync<OperationCanceledException>(async () =>
+            {
+                Task<string> readToEndTask = reader.ReadToEndAsync(token);
+
+                // This is a time-sensitive test where the cancellation needs to happen before the async read completes.
+                // A sleep may be too long a delay, so spin-wait for a very short duration before canceling.
+                SpinWait spinner = default;
+                while (!spinner.NextSpinWillYield)
+                {
+                    spinner.SpinOnce(sleep1Threshold: -1);
+                }
+
+                cts.Cancel();
+                await readToEndTask;
+            });
             Assert.Equal(token, ex.CancellationToken);
         }