Fix the APR_INCOMPLETE failures in our test suite when joining the
test threads and don't show bogus return codes upon failure.

* subversion/tests/svn_test_main.c
  (test_thread): Explicitly end the test thread.  This seems to be the
                 Right Thing to do for non-detached threads.
  (do_tests_concurrently): Make sure the return value has been initialized
                           before checking it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1546712 13f79535-47bb-0310-9956-ffa450edef68