Better ant / junit interaction

Currently junit test sources are always rebuilt, that's wasteful. The
second dependency on the junit task is there so that the first can be
skipped if sources haven't changed.

Also normalize package names versus location in the `test/junit` folder:
ant isn't very clever when it comes to selectively recompiling tests, so
now editing a test will only cause that one to be recompiled (instead of
~13 files every time).

This makes TDD with junit even faster, /cc @som-snytt :)