Improve unit tests

[jradtilbrook]

The way the unit tests make assertions to check behaviour is not very good. It relies on the phpunit configuration forcing a test to fail if it makes no assertions. It should instead bind a local variable to the closures by reference, modify it in the closure and assert it has changed. Or perhaps there is an even better way than that.

[tspencer244]

See #9 for my proposed solution. Also I think you forgot to tag this issue for Hacktoberfest!