Unmask dexter debuginfo tests on Darwin

These tests almost certainly work on Darwin anyway, I just wanted to
keep things in a fixed, working configuration, while pushing Dexter
up.

I've left Windows unsupported as the dexter command line will need further
adjustment to run dbgeng. This can be abstracted through the %dexter
substitution, but is a task for another time.

GitOrigin-RevId: 2665f3e4364f4c3aca155e16070315f059a21b41
8 files changed