Bump MSTest.TestAdapter from 4.1.0 to 4.2.1 (#427)

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

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

_Sourced from [MSTest.TestAdapter'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).
</details>

[![Dependabot compatibility
score](https://dependabot-badges.githubapp.com/badges/compatibility_score?dependency-name=MSTest.TestAdapter&package-manager=nuget&previous-version=4.1.0&new-version=4.2.1)](https://docs.github.com/en/github/managing-security-vulnerabilities/about-dependabot-security-updates#about-compatibility-scores)

Dependabot will resolve any conflicts with this PR as long as you don't
alter it yourself. You can also trigger a rebase manually by commenting
`@dependabot rebase`.

[//]: # (dependabot-automerge-start)
[//]: # (dependabot-automerge-end)

---

<details>
<summary>Dependabot commands and options</summary>
<br />

You can trigger Dependabot actions by commenting on this PR:
- `@dependabot rebase` will rebase this PR
- `@dependabot recreate` will recreate this PR, overwriting any edits
that have been made to it
- `@dependabot show <dependency name> ignore conditions` will show all
of the ignore conditions of the specified dependency
- `@dependabot ignore this major version` will close this PR and stop
Dependabot creating any more for this major version (unless you reopen
the PR or upgrade to it yourself)
- `@dependabot ignore this minor version` will close this PR and stop
Dependabot creating any more for this minor version (unless you reopen
the PR or upgrade to it yourself)
- `@dependabot ignore this dependency` will close this PR and stop
Dependabot creating any more for this dependency (unless you reopen the
PR or upgrade to it yourself)


</details>

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

Author: dependabot[bot]

Directory.Packages.props

@@ -14,7 +14,7 @@
     <PackageVersion Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="10.0.202" />
     <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.TestAdapter" Version="4.2.1" />
     <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 -->