Fix the serve-precommit target.

When the plovr-jar is being invoked in the serve-precommit target, it needs
the JSON files in `test/` as well to have all requirements for headless testing.

This should also reenable running the testsuite on our continuous integration
server.

Please also see this discussion: https://groups.google.com/forum/#!topic/ol3-dev/JBRoGIiTdvs