Bump Microsoft.NET.Test.Sdk from 16.8.3 to 16.9.1 in /Rx.NET/Source

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 16.8.3 to 16.9.1.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](https://github.com/microsoft/vstest/compare/v16.8.3...v16.9.1)

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

Rx.NET/Source/tests/Tests.System.Reactive.ApiApprovals/Tests.System.Reactive.ApiApprovals.csproj

@@ -25,7 +25,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.1" />
     <PackageReference Include="Verify.Xunit" Version="10.9.1" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />

Rx.NET/Source/tests/Tests.System.Reactive/Tests.System.Reactive.csproj

@@ -16,7 +16,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />    
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.1" />    
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
     <ProjectReference Include="..\..\src\System.Reactive\System.Reactive.csproj" />