appiumtests: Introduce a generic plasmoidtest script

A bunch of the appiumtests are simple smoke tests that just check if a plasmoid loads by trying to find a certain element by name. We can do a generic version of this by passing the plasmoid ID and the name to find on the command line, allowing us to drop a bunch of duplicated code.

This MR adds a generic script and uses it for the existing smoke tests as well as some new ones for system monitor plasmoids.