Add test to beacon node fallback feature

[chong-he]

Add test to beacon node fallback using mockito crate: https://crates.io/crates/mockito to create mock beacon nodes. The tests include:

• testing the update_all_candidates function in update_all_candidates_should_update_sync_status
• testing the broadcast function in broadcast_should_send_to_all_bns
• testing the first_success function in first_success_should_try_nodes_in_order

A majority part of the code was written by @jimmygchen, thank you Jimmy for the help and guidance, I learn a lot along the way.

[jimmygchen]

@chong-he Nice, I've added some comments but looking good!

[jimmygchen]

Thanks @chong-he, looks good to me now.

[michaelsproul]

Cleaner and simpler than I thought it would be! Nice work!

[mergify]

This pull request has merge conflicts. Could you please resolve them @chong-he? 🙏

[jimmygchen]

Great work, thanks @chong-he ☺️