* bpo-29512: Add test.bisect, bisect failing tests (#2452)

Add a new "python3 -m test.bisect" tool to bisect failing tests.

It can be used to find which test method(s) leak references, leak
files, etc.
(cherry picked from commit 84d9d14a)

* bpo-30776: regrtest: reduce memleak false positive (#2484)

Only report a leak if each run leaks at least one memory block.
(cherry picked from commit beeca6e1)