target_config: add art-test-javac target

Effectively runs the same commands as:
  ANDROID_COMPILE_WITH_JACK=false make test-art-host

This builds libcore/art with javac/dx/desugar, then runs the gtests
and run-tests for it.

(It does not yet fix run-test to use javac everywhere. That will be a
follow-up change).

Bug: 36902714
Change-Id: Ibd226efcfe78c795474f98d69f513648cff5351a