Bump Microsoft.NET.Test.Sdk from 16.0.1 to 16.1.1 in /Rx.NET/Source

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 16.0.1 to 16.1.1.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](https://github.com/microsoft/vstest/compare/v16.0.1...v16.1.1)

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

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

@@ -35,7 +35,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.8.0-preview-20180610-02" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.1.1" />
     <PackageReference Include="xunit" Version="2.4.0-beta.2.build4010" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0-beta.2.build4010" />
     <PackageReference Include="ApprovalTests" Version="3.0.14" />

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

@@ -17,7 +17,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.0.1" />    
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.1.1" />    
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
     <ProjectReference Include="..\..\src\System.Reactive\System.Reactive.csproj" />