bpo-31067: test_subprocess calls reap_children() (#2931)

test_subprocess now also calls reap_children() in tearDown(), not
only on setUp().

Lib/test/test_subprocess.py

@@ -56,6 +56,8 @@ def tearDown(self):
             inst.wait()
         subprocess._cleanup()
         self.assertFalse(subprocess._active, "subprocess._active not empty")
+        self.doCleanups()
+        support.reap_children()
 
     def assertStderrEqual(self, stderr, expected, msg=None):
         # In a debug build, stuff like "[6580 refs]" is printed to stderr at