Fixed broken tests and made minitest exit simpler.

By converting the `MiniTest::Unit.runner.run` return value to an
integer, you don't have to perform any complex logic around the exit
code because `nil.to_i == 0`. Also, because `exit` was never explicitly
called, the tests were failing.
Merge pull request #338 from panthomakos/fix-tests

Fixed broken tests and made minitest exit simpler.