Add a test for issue #4307, 'upgrade from 1.7 crashes if WC contains conflict'.

* subversion/tests/cmdline/upgrade_tests.py
  (upgrade_from_1_7, test_list): New test. Currently upgrades a WC that
   contains a text conflict, but can later be extended to cover more cases
   once issue #4307 is fixed.

* subversion/tests/cmdline/upgrade_tests_data/upgrade_from_1_7_wc.tar.bz2: New.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1439502 13f79535-47bb-0310-9956-ffa450edef68