bpo-40135: Fix multiprocessing test_shared_memory_across_processes() (GH-19892)

Don't define shared memory block's name in test_shared_memory_across_processes():
use SharedMemory(create=True) instead.
(cherry picked from commit caa3ef2)

Co-authored-by: Hai Shi <[email protected]>

Author: miss-islington

Lib/test/_test_multiprocessing.py

@@ -3828,7 +3828,9 @@ class OptionalAttachSharedMemory(shared_memory.SharedMemory):
         sms.close()
 
     def test_shared_memory_across_processes(self):
-        sms = shared_memory.SharedMemory('test02_tsmap', True, size=512)
+        # bpo-40135: don't define shared memory block's name in case of
+        # the failure when we run multiprocessing tests in parallel.
+        sms = shared_memory.SharedMemory(create=True, size=512)
         self.addCleanup(sms.unlink)
 
         # Verify remote attachment to existing block by name is working.