Make test execution time more predictable / consistent.
Make test execution time more predictable / consistent.

On machines with more than 4 HW threads, use about #HwThreads/2
threads / processes. On smaller machines, reduce the number of
iterations.

If no efficient synchronization is available, run the concurrency
tests for a very small and fixed number of iterations and on 2
cores only.

* subversion/tests/libsvn_subr/named_atomic-test.c
  (calibrate_concurrency): see above.
  (calibrate_iterations): skip runs with small numbers of iterations

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1326683 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion