Symbolic CHPL_RT_NUM_THREADS_PER_LOCALE values: MAX_PHYSICAL and MAX_LOG...

...ICAL.

Add support for two symbolic values for CHPL_RT_NUM_THREADS_PER_LOCALE:
'MAX_PHYSICAL' and 'MAX_LOGICAL'.  These will be interpreted as the
number of physical and logical CPU instances available to the program,
respectively.  Also, add unit tests for these values and as part of
this, introduce a new section of tests/ to contain runtime-specific
tests.
It seems a shame to not be able to test across platforms, but I can't think of a clean way to get the information without duplicating the code that's in the runtime to get it already. I was hoping python might already have a cross platform way to get it, but they only support getting the number of virtual cores, and it's not that portable.