Issue #6598: Avoid clock wrapping around in test_make_msgid_collisions.

Use time.monotonic instead of time.clock.

Lib/test/test_email/test_email.py

@@ -3179,7 +3179,7 @@ Foo
                 self.msgids = []
                 append = self.msgids.append
                 make_msgid = utils.make_msgid
-                clock = time.clock
+                clock = time.monotonic
                 tfin = clock() + 3.0
                 while clock() < tfin:
                     append(make_msgid(domain='testdomain-string'))