MDLSITE-3282 don't complain about plain test methods

It has become sort of a de-facto standard to have no phpdocs on test
methods in Moodle phpunit tests. This makes a lot of sense because:

* The test name should be the descriptive part - as it also is the most
 commonly shared part of any failure
* Extensive phpdoc blocks that they easily get outdated as someone adds
 another few assertions to address whats going