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)
----------------------------------------------------------------------
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

Author: vstinner

Lib/test/_test_multiprocessing.py

@@ -1204,7 +1204,7 @@ def test_put(self):
     @classmethod
     def _test_get(cls, queue, child_can_start, parent_can_continue):
         child_can_start.wait()
-        #queue.put(1)
+        queue.put(1)
         queue.put(2)
         queue.put(3)
         queue.put(4)
@@ -1229,15 +1229,16 @@ def test_get(self):
         child_can_start.set()
         parent_can_continue.wait()
 
-        time.sleep(DELTA)
+        for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
+            if not queue_empty(queue):
+                break
         self.assertEqual(queue_empty(queue), False)
 
-        # Hangs unexpectedly, remove for now
-        #self.assertEqual(queue.get(), 1)
+        self.assertEqual(queue.get_nowait(), 1)
         self.assertEqual(queue.get(True, None), 2)
         self.assertEqual(queue.get(True), 3)
         self.assertEqual(queue.get(timeout=1), 4)
-        self.assertEqual(queue.get_nowait(), 5)
+        self.assertEqual(queue.get(), 5)
 
         self.assertEqual(queue_empty(queue), True)