Add unit tests for SceneMultiplayer

[pafuent]

This PR aims to help "fix" #43440

I implemented as many test I could without using an actual MultiplayerPeer (like ENetMultiplayerPeer) or writing a MultiplayerPeer mock.
Using ENetMultiplayerPeer in this tests will imply that changes on ENetMultiplayerPeer could break tests on SceneMultiplayer, which is not desirable. Also I'm not sure if I'll have enough control on how ENetMultiplayerPeer behaves to fully test SceneMultiplayer.
For the second option, I'll wait until a decision is made on #95224. If a mocking library is available, I'll revisit SceneMultiplayer to add more unit tests.

Also I'm fixing a small typo on SceneMultiplayer docs.

[Faless]

LGTM, nice work 🚀

[mhilbrunner]

Looks good! Docs change is fine, as are the tests, built and tested locally on Windows.

[akien-mga]

Thanks!