Align test reporting more closely w/ status enums (as it related to "untested")

[cmcginley-splunk]

• the difference between "skipped" and "untested" is not obvious to the unitiated
• we should:
  • add a new UNSET status enum that is the default for TestResult.status (also good in that we no longer need to worry about the None type)
  • eliminate total_untested and untested_detections
  • If testing fails in an unhandled way that causes contentctl to stop altogether, then we iterate over all tests, and for any that remain UNSET after testing completes, we mark the status as ERROR with a message like TEST FAILED (ERROR): testing end state failed to be set, testing may have ended prematurely