Update cobegin stacksize

This test was originally added with the following message:

"Add a test that fails (on linux systems, anyway) unless the new callStackSize
config const is set at execution time, as it is in the .execopts file here."

However the failure mode really depends on the configuration (--baseline,
default, --fast) and exactly what code we're generating. Currently this tests
passes with default without setting CHPL_RT_CALL_STACK_SIZE so it's not really
testing what it intended to anymore (which I think is just that
CHPL_RT_CALL_STACK_SIZE actually changes the call stack size beyond just what
the tasking layer reports to us.)

To make it more portable to different configurations this updates so the test
runs in two modes. One with 2K which will always fail, and one with 2MB
which should always pass.

The problem size was updated to use these stack sizes, and the stack sizes were 
chosen so the test works on all our current platforms, including Cygwin where
the current default and max stack size are 2M