Feat: added tests for signature checker functions

[etherhood]

While writing tests, came across incorrect implementation of validate_signature, i have fixed it.

For testing, it was crucial to add key_pair in Mocks struct to generate signature for orders, so i have added it as well to Mock struct, but it required to remove Serde attribute, as we were not serializing and deserializing it anywhere. If it is needed I can add it and find some other way to use key_pair across tests

[0xandee]

Hi @psychiller do you mind have a check at the failed CI, really appreciate it!

[etherhood]

Fixed marketplace tests as well