multiprocessing test_shared_memory_recreate is not concurrency-safe

[tiran]

Bug report

The test cases

• test.test_multiprocessing_spawn.WithProcessesTestSharedMemory.test_shared_memory_recreate
• test.test_multiprocessing_fork.WithProcessesTestSharedMemory.test_shared_memory_recreate
• test.test_multiprocessing_forkserver.WithProcessesTestSharedMemory.test_shared_memory_recreate

are not concurrency-safe because they all use the same semaphore name. A left-over semaphore from a previous run can also cause problems The bug is currently causing several buildbots to break. The obvious solution is to use the _new_shm_name helper like the other tests.