More fun with JUnit 5

[juleskers]

I've been doing some code-coverage-guided test-writing this weekend.
This unveiled some gaps in my previous Melody and SongPlayer tests.

I've also started working my way through Piano.
It feels a bit dirty, because I had to make some edits to "production" logic, in order to make it testable.
I've been taught that's normally a big no-no (at work).
For PianOli, I'm willing to break that rule, since I think it's an intermediate step on the way to move sound-playing to PianoCanvas.
The Canvas does all the other Android-API-related stuff, while Piano is pure coordinate/state-handling if you ignore sounds.

I'm thinking Piano will end up with some kind of Listener-pattern, where both the Canvas and the AppConfigTrigger listen for keydown-events. However, it's still crystallizing in my brain.

In the meanwhile, I wanted to publish these additional tests, since they're useful in and of themselves.

[nicolasbrailo]

Thanks @juleskers
One suggestion for a test: can we add one to test the stability of the sizes? I noticed the size of the keys in the app change after the first keypress on the latest version of the app, and I'm not sure why or when we introduced that