Improve parallellism in debug test Makefile (#223)

* Improve parallellism in debug test Makefile

Now each test is an individual make target, so you can get the most out
of however many cores you have. On my 12-core system, `make` went from
2m45s to 42s, and `make all` went from `3m25s` to `2m39s`.

If you have few cores, this change may actually slow things down a bit,
because ExamineTarget is run for every gdbserver.py invocation.

* Remove test target.