[NFC][TSan][libdispatch] Fix test for dispatch_apply[_f]

* Array index out of bounds: 100 iterations, but size of array is 2.
* Unmatched barrier_init (2) with barrier_wait (200)
* Number of iterations must be smaller than the available parallelism
  for the queue, otherwise we deadlock (since every barrier_wait call
  blocks the thread).

Scary: All of this worked reliably in gcd-apply.mm (for Darwin)

Rievewed By: kubamracek

Differential Revision: https://reviews.llvm.org/D59510

llvm-svn: 356418