bpo-37359: Add --cleanup option to python3 -m test (GH-14332)

* regrtest: Add --cleanup option to remove "test_python_*" directories
  of previous failed test jobs.
* Add "make cleantest" to run "python3 -m test --cleanup".
diff --git a/Lib/test/libregrtest/cmdline.py b/Lib/test/libregrtest/cmdline.py
index dc0d880..9f1bf68 100644
--- a/Lib/test/libregrtest/cmdline.py
+++ b/Lib/test/libregrtest/cmdline.py
@@ -272,8 +272,10 @@
     group.add_argument('--junit-xml', dest='xmlpath', metavar='FILENAME',
                        help='writes JUnit-style XML results to the specified '
                             'file')
-    group.add_argument('--tempdir', dest='tempdir', metavar='PATH',
+    group.add_argument('--tempdir', metavar='PATH',
                        help='override the working directory for the test run')
+    group.add_argument('--cleanup', action='store_true',
+                       help='remove old test_python_* directories')
     return parser