Fix multiprocessing queue test_get() (GH-142024)

* Replace sleep() with support.sleeping_retry().
* Test get_nowait() first.
* Restore previously disabled test.

Fix the failure:

FAIL: test_get (test.test_multiprocessing_spawn.test_processes.WithProcessesTestQueue.test_get)
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Traceback (most recent call last):
  File "Lib/test/_test_multiprocessing.py", line 1208, in test_get
    self.assertEqual(queue_empty(queue), False)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: True != False