[3.8] bpo-45310: Fix parrallel shared memory tests (GH-28661)

[vstinner]

Add a PID to names of POSIX shared memory objects to allow
running multiprocessing tests (test_multiprocessing_fork,
test_multiprocessing_spawn, etc) in parallel.

(cherry picked from commit eb4495e)

https://bugs.python.org/issue45310

[vstinner]

s390x Fedora Refleaks 3.8 is affected by https://bugs.python.org/issue45310 bug:
https://buildbot.python.org/all/#/builders/251/builds/182

0:05:07 load avg: 5.50 [170/423/1] test_multiprocessing_spawn failed (2 min 6 sec) -- running: test_io (2 min 54 sec)
beginning 6 repetitions
123456
test test_multiprocessing_spawn failed -- Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.8.edelsohn-fedora-z.refleak/build/Lib/test/_test_multiprocessing.py", line 3753, in test_shared_memory_basics
    sms = shared_memory.SharedMemory('test01_tsmb', create=True, size=512)
  File "/home/dje/cpython-buildarea/3.8.edelsohn-fedora-z.refleak/build/Lib/multiprocessing/shared_memory.py", line 102, in __init__
    self._fd = _posixshmem.shm_open(
FileExistsError: [Errno 17] File exists: '/test01_tsmb'

I propose to backport Serhiy's test fix, since Python 3.8 buildbots are still up and running.