[OMPT] Fix test parallel/not_enough_threads.c

Upcoming changes to FileCheck will modify CHECK-DAG to not match
overlapping regions of the input. This test was found to be affected
because it expects to find four threads to invoke events of type
ompt_event_implicit_task_begin. It turns out this is wrong because
OMP_THREAD_LIMIT is set to 2, so there are only two threads. The
rest of the test got it right so it went unnoticed until now.

(Rewrite test and apply clang-format to it as discussed in the past.)

Differential Revision: https://reviews.llvm.org/D47119

llvm-svn: 333361