Fixed broken tests and made minitest exit simpler.

Tests are failing.  This resolves the test issues.  I think the primary reason was [this commit](https://github.com/panthomakos/zeus/commit/36813824).

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.