Run test job on MRs and master branch only

Configure the CI to only start once a user opens a Merge Request (can still be a draft of course) for their production.

This is helpful as it reduces the number of pipelines created to only those with MRs (i.e. some intention is there to submit the production), and means a branch being tested will always have an associated MR to post information to (see further reasoning in !128 (comment 4617982)).

I will test this in another branch rebased off this one