Remove dummy tests, Remove ActionButtonModel

[chFlorian]

Hey Trent!

In this PR I have removed ActionButtonModel, because I don't see any usage for it being an independent data type.
Instead, I have added associated values to the three cases of ActionButtonState: title, systemImage

This way the API is quite a bit easier to read:

@State var buttonState: ActionButtonState = .enabled(title: "Hello, World!", systemImage: "house")

For comparison, this is the old API:

@State var buttonState: ActionButtonState = .enabled(.init(title: "Hello, World!", systemImage: "house"))

What do you think?

I also removed the test target, because there were no actual tests.

[trentguillory]

I like it! This is much more readable. And thanks for collapsing that switch statement in ActionButtonState. It's much clearer.