Bump MSTest.TestFramework (#23060)

Bumps [MSTest.TestFramework](https://github.com/microsoft/testfx) from 2.1.2 to 2.2.1.
- [Release notes](https://github.com/microsoft/testfx/releases)
- [Commits](microsoft/testfx@v2.1.2...v2.2.1)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

samples/snippets/csharp/getting_started/ClassLibraryProjects/StringLibraryTest/StringLibraryTest.csproj

@@ -9,7 +9,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
     <PackageReference Include="MSTest.TestAdapter" Version="2.2.1" />
-    <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.2.1" />
     <PackageReference Include="coverlet.collector" Version="3.0.2" />
   </ItemGroup>