> Don't the tests just run on the branch before the merge?
The merge button is only green if github could generate a merge commit, and that merge commit is made available. You can run your tests on either the branch before merge or the branch after merge, as you prefer.
You may want to do both and have the former block the latter: the merged head is going to change (and require a re-test) any time the target branch gets a new commit, no point in wasting cycle if the branch's own tests don't pass in the first place.
The merge button is only green if github could generate a merge commit, and that merge commit is made available. You can run your tests on either the branch before merge or the branch after merge, as you prefer.
You may want to do both and have the former block the latter: the merged head is going to change (and require a re-test) any time the target branch gets a new commit, no point in wasting cycle if the branch's own tests don't pass in the first place.