Adding tests to the plugin

[OpherV]

Based on the discussion in #200 , I would love to have some automated tests for basic functionality of the plugin and its UI.

[CC007]

This could also be used for regression tests and checks for deprecation in the IntelliJ API and/or git flow API. That way it would be easier to change to a new api version and check for compatibility or regressions.

I found this page on testing plugins: https://plugins.jetbrains.com/docs/intellij/testing-plugins.html