Bump Microsoft.NET.Test.Sdk from 16.9.1 to 16.9.4

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 16.9.1 to 16.9.4.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](microsoft/vstest@v16.9.1...v16.9.4)

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

Author: dependabot[bot]

src/Directory.Build.props

@@ -31,7 +31,7 @@
         <PackageVersion_MicrosoftTextEncoding>5.0.0</PackageVersion_MicrosoftTextEncoding>
         <PackageVersion_MsBuild>16.9.0</PackageVersion_MsBuild>
 
-        <PackageVersion_MicrosoftTestSdk>16.9.1</PackageVersion_MicrosoftTestSdk>
+        <PackageVersion_MicrosoftTestSdk>16.9.4</PackageVersion_MicrosoftTestSdk>
         <PackageVersion_NSubstitute>4.2.2</PackageVersion_NSubstitute>
         <PackageVersion_CoverletMsBuild>3.0.3</PackageVersion_CoverletMsBuild>
         <PackageVersion_NUnit>3.13.1</PackageVersion_NUnit>