Reinstate rule that avoids posting test reports on non-PR pushes

[lunkwill42]

3e434bc#diff-208a7f7ef0a1174d68708f24502994bd2a136c75128806eb0cf63c0a70679321 removed an if-statement attached to the publish-tests-results workflow whose function was too unclear.

This is now causing the CI suite to fail on pushed directly to branches (like master): https://github.com/Uninett/nav/actions/runs/8155376451/job/22290756342

The point of the original logic was that when a commit is pushed outside of a PR context, there is no place to actually publish the test result report, as the action we use for this will publish the report in a PR comment.

The existing logic relied on the fact that NAV developers mostly fork the NAV repo and post PRs from their forks, instead of adding branches directly to the official repo. The only "author" that creates PRs from branches on the official repo is dependabot.

We should either:

1. Reinstate the pre-existing logic, but add a comment explaining why it's there

or

2. Find some way to instead detect if the workflow is running as a result of a PR rather than a plain push to a branch and add that logic instead.

[lunkwill42]

Reposting my comment from #2922:

Actually, I think we should maybe revert #2914 fully, and put #2862 on standby.

It seems that the test result publisher is indeed able to publish test results on the actions page when there is no PR associated with a commit. It's not that we want to avoid the test report, we want the reports - it's just that we thought there was nowhere to actually post it when there was no PR.

The confusing factor here is that the action fails, even if the results are published - apparently as it is trying to find a PR associated with the commit hash of the triggering commit.

Example in question: https://github.com/Uninett/nav/actions/runs/9063635622