Add missing step (dotnet#25437)

Author: tdykstra

docs/core/testing/unit-testing-with-dotnet-test.md

@@ -3,7 +3,7 @@ title: Unit testing C# code in .NET Core using dotnet test and xUnit
 description: Learn unit test concepts in C# and .NET Core through an interactive experience building a sample solution step-by-step using dotnet test and xUnit.
 author: ardalis
 ms.author: wiwagn
-ms.date: 10/21/2020
+ms.date: 08/02/2021
 ---
 # Unit testing C# in .NET Core using dotnet test and xUnit
 
@@ -81,9 +81,10 @@ The following instructions provide the steps to create the test solution. See [C
 * The preceding command:
   * Creates the *PrimeService.Tests* project in the *PrimeService.Tests* directory. The test project uses [xUnit](https://xunit.net/) as the test library.
   * Configures the test runner by adding the following `<PackageReference />`elements to the project file:
-    * "Microsoft.NET.Test.Sdk"
-    * "xunit"
-    * "xunit.runner.visualstudio"
+    * `Microsoft.NET.Test.Sdk`
+    * `xunit`
+    * `xunit.runner.visualstudio`
+    * `coverlet.collector`
 
 * Add the test project to the solution file by running the following command:
 
@@ -167,7 +168,8 @@ Run `dotnet test`. The test passes.
 
 ### Add more tests
 
-Add prime number tests for 0 and -1. You could copy the preceding test and change the following code to use 0 and -1:
+Add prime number tests for 0 and -1. You could copy the test created in the preceding step and make copies of the following code to test 0 and -1.
+But don't do it, as there's a better way.
 
 ```csharp
 var primeService = new PrimeService();
@@ -196,11 +198,15 @@ public void IsPrime_InputIs1_ReturnFalse()
 
 with the following code:
 
-[!code-csharp[Sample_TestCode](../../../samples/snippets/core/testing/unit-testing-using-dotnet-test/csharp/PrimeService.Tests/PrimeService_IsPrimeShould.cs?name=Sample_TestCode)]
+ :::code language="csharp" source="../../../samples/snippets/core/testing/unit-testing-using-dotnet-test/csharp/PrimeService.Tests/PrimeService_IsPrimeShould.cs" id="Sample_TestCode":::
 
 In the preceding code, `[Theory]` and `[InlineData]` enable testing several values less than two. Two is the smallest prime number.
 
-Run `dotnet test`, two of the tests fail. To make all of the tests pass, update the `IsPrime` method with the following code:
+Add the following code after the class declaration and before the `[Theory]` attribute:
+
+ :::code language="csharp" source="../../../samples/snippets/core/testing/unit-testing-using-dotnet-test/csharp/PrimeService.Tests/PrimeService_IsPrimeShould.cs" id="Sample_InitCode":::
+
+Run `dotnet test`, and two of the tests fail. To make all of the tests pass, update the `IsPrime` method with the following code:
 
 ```csharp
 public bool IsPrime(int candidate)

samples/snippets/core/testing/unit-testing-using-dotnet-test/csharp/PrimeService.Tests/PrimeService_IsPrimeShould.cs

@@ -6,12 +6,14 @@ namespace Prime.UnitTests.Services
 {
     public class PrimeService_IsPrimeShould
     {
+        #region Sample_InitCode
         private readonly PrimeService _primeService;
 
         public PrimeService_IsPrimeShould()
         {
             _primeService = new PrimeService();
         }
+        #endregion
 
         #region Sample_TestCode
         [Theory]