Improve localizing ResourceWarnings

[d-maurer]

ResourceWarnings are often issued during garbage collection. Those reports are non local and therefore it is difficult to find the resource's origin.

Python's tracemalloc allows to keep track of allocation tracebacks and if recorded such a traceback is included in the ResourceWarning. But the report is still not associated with a particular test, and it is often not easy to properly configure tracemalloc (regarding traceback depth) to reliably obtain the information which test has caused the problem.

I propose an new option --gc-after-test. If set, gc.collect is called after each test (similar to --report-refcounts); this way, a ResourceWarning can be easily associated with the test revealing the resource problem.

Opinions?

[jugmac00]

Sounds awesome!