Abort tests when instances leaked

[balloob]

Description:

This will abort the test run when instances are leaked instead of raising an asserting error for this and every subsequent error (which pretty much ruins the logs).

Most of the times these errors are because our detection for calling a threaded helper from inside async is accidentally triggered. Not sure yet how to get around these false positives.

Checklist:

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[mention-bot]

@balloob, thanks for your PR! By analyzing the history of the files in this pull request, we identified @fabaff, @MrMep and @kellerza to be potential reviewers.