Bump Microsoft.NET.Test.Sdk from 17.2.0 to 17.3.1 (#786)

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 17.2.0 to 17.3.1.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](https://github.com/microsoft/vstest/compare/v17.2.0...v17.3.1)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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tests/UnitTests/UnitTests.csproj

@@ -11,7 +11,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.2.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.2.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.3.1" />
     <PackageReference Include="Moq" Version="4.18.2" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">