Devise tests

[Pharap]

This library's gone a long time without a proper set of tests.

At some point in the future it would be nice to have a set of tests that could be used to ensure the code is working correctly and nothing gets broken during bugfixes.

[ruifig]

If you still plan to go ahead with this, I can suggest my own unit test library: https://github.com/ruifig/czmut