Make the cleanup behavior of the C tests when running parallel similar to
cleaning up in a single threaded run: after every test.

As the function for registering cleanup paths doesn't have any usable
argument for finding a per thread pool, introduce a per thread variable in
the test runner.

This reduces the amount of temporary storage used by our testsuite enough
to finally allow running the BDB tests without creating a bigger ramdrive
or switching to non-parallel builds.

* subversion/tests/svn_test_main.c
  (thread_local): Define on most platforms that don't define this C11 type.
  (cleanup_pool): Make thread local if possible.
  (test_thread): Create and clear a per thread cleanup pool if necessary.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1582396 13f79535-47bb-0310-9956-ffa450edef68