[CHECKLIST] Relayer Unit Testing in Rust

[shekohex]

Overview

To improve the relayer testing and code coverage, we are aiming to write all of our tests in Rust, that would allow us for writing separate and small unit tests + faster integration tests.

This a checklist of different small tasks that should be done to achieve that goal.

Checklist

• [x] #392
• [x] #394
• [x] #395
• [x] #396
• [x] #397
• [x] #398
• [x] #399

Please note that this Checklist is not final, there are more tasks to be added here.