tests: Improve test bed

[Totodore]

• Add a custom new_dummy_socket with tx/rx to send and receive packet without creating a real connexion.
• Remove dep socket_io client.
• Remove the feature test-utils in favor of a rust cfg flag socketioxide-test
• Move all the examples to a separate workspace to avoid dependency bloat / useless checks when working on socketioxide. A separate job actions in now dedicated to checking the example code.