Reduce the time spent in named atomic tests.  Modern multi-core CPUs still
execute 10s of thousands of iterations even with the reduced timings.

* subversion/tests/libsvn_subr/named_atomic-test.c
  (calibrate_iterations): quicker calibration; reduce actual test for .2 secs
  (calibrate_concurrency): 8x concurrency is good enough as a limit

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544730 13f79535-47bb-0310-9956-ffa450edef68