Include "how to test" section in the MR template

As GitLab gets bigger and bigger it is sometimes quite difficult for reviewers to figure out how a particular change can be tested when it is outside of their expertise.

I suggest we include "How to test" section in the MR template. It wouldn't be necessary to fill in the section in every MR, but when the MR includes things where settings have to be changed, data in db checked or specific commits to be pushed it would be really useful.