Lower stack size for callStackSize test

This test is intended to show that the runtime changes stack size based off of
CHPL_RT_CALL_STACK_SIZE. It used to set the value to a little over 12MB.
However on cygwin our default and max stack size are 2MB so this just lowers
the value set to a little under 1MB. It doesn't really matter what the value is
so long as it's different than the default, and it's lower than the max across
different configurations.