Do not run the C tests when testing ra_svn or ra_serf.

* build/run_tests.py
  (TestHarness.__init__): Check if we're using the file:// protocol.
  (TestHarness.run): Filter the C tests out of the test list if we're not
   testing ra_local.
  (TestHarness._split_nums): New; code hoisted out of TestHarness._run_test.
  (TestHarness._run_test): Use TestHarness._split_nums.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1609477 13f79535-47bb-0310-9956-ffa450edef68