Task should throw an error when no tests are discovered

[bryanbcook]

While #98 refers to throwing errors when test result files are not found, the PublishTestPlanResults task should, at a minimum, warn when no tests were resolved from the test results.

As an enhancement, the task could support input parameters similar to the PublishTestResults@2's failTaskOnMissingResultsFile:

• failTaskOnMissingResultsFile:

  • throw an error if no test result files are found.
  • the default for PublishTestResults@2 is false. The task presently fails when no test results are found so if we default the value to false this may break compatibility expectations if teams are using this task to ensure test results are present.

• failTaskOnMissingTests:

  • convert the warning of zero tests into a failure.
  • should default to false as this is the current behaviour.

• failTaskOnUnmatchedTestCases:

  • throw an error if the matching strategy produced no matching results
  • should default to true as this is the current behaviour.