[3.5] bpo-29512, bpo-30764, bpo-30776: Backport regrtest enhancements from 3.6 to...

[3.5] bpo-29512, bpo-30764, bpo-30776: Backport regrtest enhancements from 3.6 to 3.5 (#2540)

* [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)

(cherry picked from commit a3ca94d0504157a112a1f89bfe8be1307116fc73)

* bpo-30764: Fix regrtest --fail-env-changed --forever (#2536) (#2539)

--forever now stops if a fail changes the environment.
(cherry picked from commit 5e87592fd12e0b7c41edc11d4885ed7298d5063b)
(cherry picked from commit 4132adb4b8f90893d8f19ff46edff56f189faca0)