Issue #6598: Avoid clock wrapping around in test_make_msgid_collisions.

Use time.monotonic instead of time.clock.

diff --cc Lib/test/test_email/test_email.py
index dff3e4b0ca28076d9d7ebe72bfc06e880e763930,84f4e383f914a66c3ca82da2b251d25408b6ac9b..894b8003cde9352b8edd43198e189492d4171062
--- 1/Lib/test/test_email/test_email.py
--- 2/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@@ -3179,7 -3164,10 +3179,7 @@@ Fo
                  self.msgids = []
                  append = self.msgids.append
                  make_msgid = utils.make_msgid
-                 clock = time.clock
 -                try:
 -                    clock = time.monotonic
 -                except AttributeError:
 -                    clock = time.time
++                clock = time.monotonic
                  tfin = clock() + 3.0
                  while clock() < tfin:
                      append(make_msgid(domain='testdomain-string'))