Fix intermittent failure of merge_tests.py 137: 'single editor drive
notifications'.  This is the test for issue #4317 "redundant notifications
in single editor drive merge".

* subversion/tests/cmdline/merge_tests.py
  (single_editor_drive_merge_notifications): Use UnorderedOutput so
   variations in merge 'U 'pdate notifications don't cause the test to fail.
   Also tweak the doc string for the test to something a bit more
   descriptive.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1445171 13f79535-47bb-0310-9956-ffa450edef68