See below - there are tests, see
https://github.com/torchbox/wagtail/blob/master/wagtail/wagt... and https://github.com/torchbox/wagtail/commit/a77977a31cdfcdf95...
but we need more, and to make it more obvious how to run them. You're right that we have to do more to attract contributors. In the meantime we're offering t-shirts for pull requests...