Support ad-hoc testing of individual components (without a view)

[mstahv]

It would help creating simpler "ui unit tests" for re-usable components within application projects if tests would not be tied to a view. Didn't check from the code if it is currently possible with some trick, but at least not well documented.

The implementation could create (and dynamically register) an adhoc view for the given component automatically, so that it anyways behaves like it was properly "attached".

[mstahv]

@StefanPenndorf Check out #72 and give your early thoughts. An early draft by Claude Code. I'm not confident about the naming, but Claude is 😂

[mstahv]

The feature has been also requested here in comments section: https://vaadin.com/docs/latest/flow/testing/browserless/getting-started