Tests: Ensure all core features have complete unit tests

[einarf]

Seeing that we recently discovered a nasty bug in SpriteList.swap it's probably a good idea to do an additional pass on unit tests. This feature never actually had a unit test. I suspect both Sprite and SpriteList could need some checking. Possibly additional modules/classes should also be looked into. We can't test everything but testing the "inner core" well is at least mandatory.

The initial work is to map out what features have insufficient tests.

• Sprite tests needs to be split up. We make separate modules for BasicSprite and Sprite. Rendering tests must go in separate module.
• ... MORE ..

[benjamin-kirkbride]

Related: #2130

[einarf]

I did a lot of sprite unit test cleanup when looking at #2122