Bump Microsoft.NET.Test.Sdk from 16.3.0 to 16.4.0 in /Rx.NET/Source

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 16.3.0 to 16.4.0.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](microsoft/vstest@v16.3...v16.4.0)

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

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

@@ -28,7 +28,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.3.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
     <PackageReference Include="ApprovalTests" Version="4.3.0" />

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

@@ -17,7 +17,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.3.0" />    
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />    
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
     <ProjectReference Include="..\..\src\System.Reactive\System.Reactive.csproj" />