Begin converting C tests to use the same transient directory as the Python
tests, so that both can be run off a RAM disk.

* subversion/tests/svn_test.h (svn_test_data_path): New prototype.
* subversion/tests/svn_test_main.c: Include svn_utf.h.
  (data_path): New static variable.
  (init_test_data): New; initializes the value of data_path.
  (svn_test_data_path): Implement.
  (main): Call init_test_data and handle any resulting errors.

* subversion/tests/libsvn_client/client-test.c,
  subversion/tests/libsvn_delta/random-test.c,
  subversion/tests/libsvn_diff/diff-diff3-test.c: Update tests to only
   create files in the transient directory.

* subversion/tests/libsvn_client,
  subversion/tests/libsvn_diff: Do not ignore names that are no longer
   created in the source directory, even for in-tree builds.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1536700 13f79535-47bb-0310-9956-ffa450edef68