Add a regression test for the mod_dontdothat vs svn log --diff issue
reported in the 'Horrible error message on refused diff' thread on dev@s.a.o.

This is the first test that relies on mod_dontdothat being available
when running the tests and as-such I applied a skip for non-windows systems
until the buildbots are reconfigured.

* subversion/tests/cmdline/authz_tests.py
  (log_diff_dontdothat): New test.
  (test_list): Add test.
  
* win-tests.py
  (locate_libs): Find and copy mod_dontdothat.
  (Httpd.__init__): Load mod_dontdothat module.
  (_svn_repo): Generate configured directory tree.
  (_create_dontdothat_file): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464416 13f79535-47bb-0310-9956-ffa450edef68