[3.6] bpo-31178: Mock os.waitpid() in test_subprocess (GH-3896) (#3897)

Fix test_exception_errpipe_bad_data() and
test_exception_errpipe_normal() of test_subprocess: mock os.waitpid()
to avoid calling the real os.waitpid(0, 0) which is an unexpected
side effect of the test.
(cherry picked from commit 11045c9d)

Lib/test/test_subprocess.py

@@ -1568,6 +1568,8 @@ class POSIXProcessTestCase(BaseTestCase):
 
         fork_exec.side_effect = proper_error
 
+        with mock.patch("subprocess.os.waitpid",
+                        side_effect=ChildProcessError):
             with self.assertRaises(IsADirectoryError):
                 self.PopenNoDestructor(["non_existent_command"])
 
@@ -1586,6 +1588,8 @@ class POSIXProcessTestCase(BaseTestCase):
 
         fork_exec.side_effect = bad_error
 
+        with mock.patch("subprocess.os.waitpid",
+                        side_effect=ChildProcessError):
             with self.assertRaises(subprocess.SubprocessError) as e:
                 self.PopenNoDestructor(["non_existent_command"])