[OMPT] Fix inaccuracies in worksharing tests

These tests were failing rarely on my MacBook when there was some
activity in the background. Read: one of a thousand executions?

 * sections.c missed the sorting based on thread ids. This worked
   as long as the master thread finished its section before the
   worker thread started the second one but failed if the master
   thread was put to sleep by the OS.
 * The checks in single.c assumed that the master thread executes
   the single region which works most of the time because it is
   usually faster than the newly spawned worker thread.

Differential Revision: https://reviews.llvm.org/D39853

llvm-svn: 318527