Use a pool of integer objects toprevent false alarm when checking for
memory block leaks. Fill the pool with values in -1000..1000 which
are the most common (reference, memory block, file descriptor)
differences.
Co-Authored-By: Antoine Pitrou <pitrou@free.fr>

* bpo-26732: fix too many fds in processes started with the "forkserver" method

A child process would inherit as many fds as the number of still-running children.

* Add blurb and test comment

Only report a leak if each run leaks at least one memory block.

* Change the regrtest --huntrleaks checker to decide if a test file
  leaks or not. Require that each run leaks at least 1 reference.
* Warmup runs are now completely ignored: ignored in the checker test
  and not used anymore to compute the sum.
* Add an unit test for a reference leak.

Example of reference differences previously considered a failure
(leak) and now considered as success (success, no leak):

    [3, 0, 0]
    [0, 1, 0]
    [8, -8, 1]

Write also dots into stderr, instead of stdout.

file descriptors. Patch written by Richard Oudkerk.

* Fix "-m test --forever": replace _test_forever() with self._test_forever()
* Add unit test for --forever
* Add unit test for a failing test
* Fix also some pyflakes warnings in libregrtest