Fix two bugs with stack corefiles patch, restrict test built debugserver

These two tests, TestSkinnyCorefile.py and TestStackCorefile.py,
require a new debugserver on darwin systems to run correctly; for now,
skip them if the system debugserver is in use.  There's no easy way to
test if the debugserver being used supports either of these memory
region info features. For end users, the fallback will be a full
corefile and that's not the worst thing, but for the tests it is a
problem.