backport.pl: Add a test for conflicts mode.

* tools/dist/backport_tests.py
  (re): Import.
  (BackportTest.__init__, verify_backport): Permit UUID to be None.
  (run_backport): Grow ERROR_EXPECTED and EXTRA_ENV parameters, reshuffle the
    code to support them.
  (backport_conflicts_detection): New test.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592926 13f79535-47bb-0310-9956-ffa450edef68