Improve some expected-output code in the test suite.

* subversion/tests/cmdline/merge_tests.py
  (expected_merge_output): Simplify. Add a 'skipped_paths' parameter to
    complete the set of conflict summary reports that it can handle. Extract
    the summary-of-conflicts code as a new function in main.py.

* subversion/tests/cmdline/svntest/main.py
  (summary_of_conflicts): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442705 13f79535-47bb-0310-9956-ffa450edef68