backport.pl: Add regression tests.

* tools/dist/backport_tests.py: New file.

* tools/dist/backport_indented_entry.dump,
* tools/dist/backport_two_approveds.dump,
* tools/dist/backport_accept.dump,
* tools/dist/backport_branches.dump,
* tools/dist/backport_multirevisions.dump,
   New expected output files.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1592722 13f79535-47bb-0310-9956-ffa450edef68