Terminate the test suite within the callback

[smashwilson]

In atom/github#1664, I'm trying to prevent our test suite from running when the version range specified in the engines field of our package.json doesn't include the version of the host Atom process, so that we can bump the required Atom version without breaking our stable and beta test channels. This is the cleanest way I can think of to stop the test suite from running after buildAtomEnvironment() is available to query for the current version and exit with a 0 status.