TravisCI: run neko/macro/php/cpp tests

As title, run unit test for neko/macro/php/cpp targets. It works on my fork: https://travis-ci.org/andyli/haxe/builds/14654820
I will add support of other targets later.

Note that php target is currently broken, and I have put it under the `allow_failures` list in `.travis.yml`. It can be removed from the list once the target is fixed.

`tests/unit/Test.hx` was modified a bit to exit with correct exit code.