Bump MSTest.TestFramework from 4.1.0 to 4.2.1 (#423)

Updated [MSTest.TestFramework](https://github.com/microsoft/testfx) from
4.1.0 to 4.2.1.

<details>
<summary>Release notes</summary>

_Sourced from [MSTest.TestFramework's
releases](https://github.com/microsoft/testfx/releases)._

## 4.2.1

See the release notes
[here](https://github.com/microsoft/testfx/blob/main/docs/Changelog.md#​421)

Commits viewable in [compare
view](microsoft/testfx@v4.1.0...v4.2.1).
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Directory.Packages.props

@@ -15,7 +15,7 @@
     <PackageVersion Include="Microsoft.CodeAnalysis.Workspaces.MSBuild" Version="5.3.0" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
     <PackageVersion Include="MSTest.TestAdapter" Version="4.1.0" />
-    <PackageVersion Include="MSTest.TestFramework" Version="4.1.0" />
+    <PackageVersion Include="MSTest.TestFramework" Version="4.2.1" />
     <PackageVersion Include="System.CommandLine.DragonFruit" Version="0.4.0-alpha.25320.106" />
     <!-- Reference-only packages for test framework namespace resolution in analyzer tests -->
     <PackageVersion Include="xunit.v3.core" Version="3.2.2" />