Plugin self tests

[stefan6419846]

Is your feature request related to a problem?

I just built the plugin the other day, when I observed some warnings being emitted, which mostly originated from the plugin API calling some deprecated methods. Fixing these issues might not be obvious as this might have unexpected side effects which are not visible during some quick testing.

Describe the solution you'd like

The plugin provides corresponding self tests (as unit and integration/UI tests), allowing a contributor to run them after some changes to verify the correct plugin behavior.

JetBrains provides some guidance for this: https://plugins.jetbrains.com/docs/intellij/testing-plugins.html. checkstyle-idea has some tests as well, which is described as the "reference implementation" in the README (although Checkstyle is written in Java, in contrast to pylint being Python-based).

Describe alternatives you've considered

Manually testing the plugin will work for a rather basic overview, but extensive coverage of all relevant parts (in the best case) is rather unlikely in this case.