Fix a brain-fail with debuginfo-tests/dexter internal tests

I baked the "test" subcommand into the %dexter substituion, as that's
what all of the dexter tests use. However I forgot about the internal
tests for whether dexters features are working. Install a %dexter_base
command to allow those tests to call dexter.py directly, and un-xfail
the tests on darwin.

Update too the list of paths the unittests shouldn't try and cover, as it
tries to load dbgeng on unix machines. Ideally we wouldn't be using this
method of test discovery in the future.