Add some parallel_for_each tests

Add some parallel_for_each tests

Tom de Vries noticed that a patch in the DWARF scanner rewrite series
caused a regression in parallel_for_each -- it started crashing in the
case where the number of threads is 0 (there was an unchecked use of
"n-1" that was used to size an array).

He also pointed out that there were no tests of parallel_for_each.
This adds a few tests of parallel_for_each, primarily testing that
different settings for the number of threads will work.  This test
catches the bug that he found in that series.