Tests for forks can be skipped

[imilchev]

Currently, if a PR is opened from a fork the integration tests are skipped. They can be triggered by labeling the PR. This works correctly, however, it also means we can forget to run those tests. I propose to make the pipeline fail if the tests haven't run for the PR. This makes sure we cannot merge the PR without running the tests in the first place.

I took our approach and applied it to another repo. I modified it a bit there to also add a comment to the PR regarding the tests and to make sure by default the workflow is failed instead of skipped. See podkrepi-bg/frontend#1243 (comment)