enablePublishTestResults=true introduces build warnings

[MichaelSimons]

Enabling enablePublishTestResults will likely always introduce build warnings in a build. This is because it supports both TRX and XUnit test results. Unless you have both, one of the Publish TRX Test Results or Publish XUnit Test Results steps will cause a warning because no matching files were found. This is annoying for those who like clean builds and would like to avoid these warning headers from always appearing.

One way to solve this would be to expose an option for the consuming repo to specify which test results type to support. By default it could be *.