Bump MSTest from 3.6.3 to 3.6.4 in /src

Bumps [MSTest](https://github.com/microsoft/testfx) from 3.6.3 to 3.6.4.
- [Release notes](https://github.com/microsoft/testfx/releases)
- [Changelog](https://github.com/microsoft/testfx/blob/main/docs/Changelog.md)
- [Commits](microsoft/testfx@v3.6.3...v3.6.4)

---
updated-dependencies:
- dependency-name: MSTest
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

src/TestFixture.Tests/TestFixture.Tests.csproj

@@ -7,7 +7,7 @@
 
   <ItemGroup>
     <PackageReference Include="FluentAssertions" Version="7.0.0" />
-    <PackageReference Include="MSTest" Version="3.6.3" />
+    <PackageReference Include="MSTest" Version="3.6.4" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="System.Linq.Async" Version="6.0.1" />
   </ItemGroup>