After PR11772, some buildbots with refleak checks fail because the tests modify the environment (leaking processes or threads or files ...). Some of the failures:

https://buildbot.python.org/all/#builders/80/builds/506
ttps://buildbot.python.org/all/#builders/114/builds/375
https://buildbot.python.org/all/#builders/1/builds/497


== Tests result: ENV CHANGED ==
409 tests OK.
10 slowest tests:
- test_multiprocessing_spawn: 32 min 19 sec
- test_concurrent_futures: 25 min 37 sec
- test_asyncio: 24 min 56 sec
- test_multiprocessing_forkserver: 13 min 57 sec
- test_multiprocessing_fork: 11 min 2 sec
- test_zipfile: 10 min 2 sec
- test_decimal: 8 min 30 sec
- test_gdb: 8 min 301 ms
- test_lib2to3: 7 min 46 sec
- test_buffer: 7 min 43 sec
3 tests altered the execution environment:
    test_multiprocessing_fork test_multiprocessing_forkserver
    test_multiprocessing_spawn
8 tests skipped:
    test_devpoll test_kqueue test_msilib test_startfile
    test_winconsoleio test_winreg test_winsound test_zipfile64