Test failures

An unrelated test failed but passes locally

This may be a know random failure. You can read how to deal with a random failure.

A related test failed but passes locally

There are probably differences between your local test environment and CI:

• The testbot is running a different version PHP than your local.
• The testbot runs tests using run-tests.sh. Try using that technique and see if your tests passes.
• The testbot has to be able to run through the install process, then log in and visit the testing page, all using the web UI. Have you tried that?
• The testbot starts with all modules uninstalled and the tests themselves enable them. Uninstall your relevant modules so that they're not active locally before running local tests to get a reasonable result.
• Make sure the test is running on the correct version. Usually this is the HEAD of the main development branch.

Module dependencies are not loaded in the test

Dependencies must be declared in composer.json files. *.info.yml files are not used by CI to get dependencies.