Automate uploading of test results to Azure Pipelines

[jnm2]

The Cake script can now start printing this to stdout to upload per target framework rather than us hardcoding the target framework list into the yaml:

##vso[results.publish type=NUnit;mergeResults=true;config='Release';runTitle='netcoreapp2.0/Windows';publishRunAttachments=true;resultFiles=D:\a\1\s\test-results\netcoreapp2.0\nunit.framework.tests.xml,D:\a\1\s\test-results\netcoreapp2.0\nunitlite.tests.xml]

See #3012 (comment)