Add a test to merge pipeline for commits behind target branch

As discussed in call on 22.9.2022 and within mail, another test job should be added to the merge request pipeline that prohibits merging if the source branch is behind the target branch in commits. I.e., ensure that merge can be done only and only if the source branch has all the latest contents of the target branch and no automated merge resolution will be ever applied. The reason why this is wanted is that otherwise there could pop up unintended changes in the main branch due to Git not handling the merge as expected.