Run tests in CI, and fail CI if tests fail

[Leseratte10]

Now that the CI and all the test stuff is merged, here's another update for that:

With this change to the CI pipeline, all the tests will also run for each commit, and the CI build will fail if any of the tests fail. The MAKEFLAGS += k makes sure that the tests continue even if one of them fails (so everything gets tested), and the false after a failed test is needed so the final return value of make returns an error so the CI build stops.