Add tests for signal await with parameters

[KoBeWi]

You can await a signal and assign the result to a variable. It will be null, Variant or Array, depending on emitted signal parameters.

This feature doesn't seem documented anywhere and it's generally not very known, so this PR adds some tests so it's not lost at least.

[kleonc]

This feature doesn't seem documented anywhere and it's generally not very known

Note it's documented in 3.x for yielding on a signal. So for those familiar with 3.x it's kinda expectable it works like that (but indeed should be documented for await).

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BTW when getting an array from awaiting a signal it's still indistinguishable whether the signal was emitted with a single array argument or with multiple arguments. Probably should be explicitly noted when being documented.

[akien-mga]

Thanks!