Replace the thread-pool in our test main by a set of explicitly created
and terminated threads.  The job queue is a simple atomic counter.
Once at it, give every thread its separate, non-synchronizing memory pool.

* subversion/tests/svn_test_main.c
  (HAVE_THREADPOOLS): Drop macro.
  (test_params_t): This is now a singleton structure used as thread input
                   as well as for tracking test progress.
  (test_thread): Create your own root pool.  Grab test numbers and
                 execute tests until the test array has been exhausted.
  (do_tests_concurrently): Simply init the params, start N threads and
                           wait for them to finish.
  (main): Adapt to macro being dropped.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1546421 13f79535-47bb-0310-9956-ffa450edef68