Follow-up to r1609477: Make skipping the C tests optional for any RA method.

* Makefile.in (check): Interpret the SKIP_C_TESTS option.
* build/run_tests.py
  (TestHarness.__init__): Accept a new optional parameter skip_c_tests
   and initialize the object attribute from it.
  (main): Add new options --skip-c-tests and synonym --skip-C-tests.
   Defaults to false; pass the value to the TestHarness constructor.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1609489 13f79535-47bb-0310-9956-ffa450edef68