If you’re on a standard install, your “master” branch in /var/discourse is the discourse_docker repository: GitHub - discourse/discourse_docker: A Docker image for Discourse · GitHub
With a standard install, “tests-passed” is the default branch. I can’t speak to why tests aren’t passing 100% of the time, but my understanding is that the test suite passed in master once before for that branch.
TLDR, I’ve found tests-passed pretty safe.