[#1076] Warn if tests leave active threads.

Problem description
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We want to keep an eye on how threads are managed or left unmanaged.


Changes description
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The ChevahTestCase teardown was updated to check for threads.

There is an exceptions for Twisted will be fixed in another branch.


How to try and test the changes
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reviewers @alibotean 

Nothing much here. There is an associated server branch using this version of package