[3.6] bpo-29512, bpo-30776: Backport regrtest enhancements from master to 3.6 (#2513)

[3.6] bpo-29512, bpo-30776: Backport regrtest enhancements from master to 3.6 (#2513)

* 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 84d9d14a1fa395fbd21262ba195490be25a7b3bc)

* 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 beeca6e1e5fd01531b1db7059498b13d07dca525)