Add CI templates for Plasma

People are lazy to run tests. Because of this, broken tests are typically discovered long after the regression had been introduced.

This problem is very common in kwin and from time to time it re-appears in kwayland-server. The best way to deal with issues of this kind is to catch them during the code review stage, but as it's been said, people are lazy and they typically, including me, forget to run tests. GitLab provides a feature called "pipelines" that can help us with it.

While kwin is a bit nitpicky about CI, for example it doesn't like if the test runner machine is too loaded, kwayland-server is less prone to this kind of issues.