System tests are fragile for any object leaks, especially those holding
open files. Instead of wrapping all tests with try/finally removing
those local variables (as done in qubes.tests.integ.backup for example),
apply generic solution: clean all traceback objects from local
variables. Those aren't used to generate text report by either test
runner (qubes.tests.run and nose2). If one wants to break into debugger
and inspect tracebacks interactively, needs to comment out call to
cleanup_traceback.
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