Document JS test suite basics

Created by: cvrebert

Similar to the test-specific README in https://github.com/twbs/bootlint/tree/master/test , I propose we add some docs explaining the basics of the test suite. E.g. we use QUnit (link to QUnit's API docs), there's one file per JS plugin, here's how to run the unit tests, here's what the "visual" tests are, etc.