Frontend tests are flakey, sometimes inconsistent

See https://gitlab.gwdg.de/grady-corp/grady/-/pipelines/418811

Some of the frontend tests have a chance to fail on one run, but succeed on another with no changes to code or CI configuration. This is likely due to an insufficiently long waiting time for the selenium web driver on certain actions, as some element hasn't been rendered in time and thus can't be found. This hypothesis seems sensible since I've noticed that when using a local gitlab-runner, the tests seem to have a much higher success chance.

We should consider adding small waiting periods to tests which rely on interacting with elements like popup dialogues and such, where necessary.