Get netstandard tests reported in AppVeyor UI

[masojus]

Take a look at the comments here: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-test

Specifically regarding passing options to the underlying test adapter so we can customize the output format of the nunit tests for AppVeyor to display them in CI runs.

[masojus]

nunit/nunit3-vs-adapter#215

[masojus]

It also looks like the VS adapter is indeed able to discover and run the tests in the VS UI. I wonder if directly using the nunit3-console.exe can run the netstandard tests and produce XML output.

[masojus]

https://www.appveyor.com/docs/running-tests/#pushing-real-time-test-results-to-build-console

[masojus]

At the moment not even the .NET Framework tests are being reported because we run them under OpenCover to gather coverage data and then don't upload the results.xml. Before, these tests were getting reported in the UI because we were using AppVeyor's Auto test discovery/execution.

[masojus]

To upload the netstandard test results, we might have to use the trx logger and upload to AppVeyor's reporting endpoint with mstest as the resultsType: https://github.com/dotnet/cli/issues/3114#issuecomment-291104000

[masojus]

Actually n/m, the .NET Framework tests are getting automatically reported because we output a filename and format AppVeyor knows about....we just weren't actually running those tests :/ That's fixed now, so what remains will be to see how we report the .NET Standard tests if/when we get rid of the big .NET Framework tests, which will likely only happen if we can report coverage for those too.

[baumatron]

This should be working now because of #124.