Quiet gdbddash test on darwin

I could not find gdb on darwin (our test machine nor my mac), so the --gdb flag will not work on that platform.
We could have --gdb call lldb on darwin, but it appears that the approach taken was to add a parallel flag --lldb for use on platorms that support that debugger.  So in line with that design decision, it makes better sense to split the test.

So I did that.  I added a .skipif file, so the existing test does not run on darwin.
I added the new test declint-lldb that is run only on darwin and tosses the --lldb flag instead.