Bump MSTest

Bumps the dotnet group in /docs/core/tutorials/snippets/library-with-visual-studio/csharp with 1 update: [MSTest](https://github.com/microsoft/testfx).


Updates `MSTest` from 3.7.2 to 3.7.3
- [Release notes](https://github.com/microsoft/testfx/releases)
- [Changelog](https://github.com/microsoft/testfx/blob/main/docs/Changelog.md)
- [Commits](microsoft/testfx@v3.7.2...v3.7.3)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/core/tutorials/snippets/library-with-visual-studio/csharp/StringLibraryTest/StringLibraryTest.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="MSTest" Version="3.7.2" />
+    <PackageReference Include="MSTest" Version="3.7.3" />
     <PackageReference Include="coverlet.collector" Version="6.0.4">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>