Give every thread in the parallel recursive mkdir test its own pool
to play with, to prevent weird data interdependencies.

* test/testdir.c
  (struct thread_data): Encapsulates abts_case and per-thread pool.
  (thread_mkdir_func): Thread data is thread_data, not abts_case.
  (test_mkdir_recurs_parallel):
     Create a separate pool and thread data struct for each thread.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/trunk@1559975 13f79535-47bb-0310-9956-ffa450edef68