createMockedFunction: check if `fn_name` and the name of the function in `fn_signature` are the same

[dimitrovmaksim]

Currently when creating a mocked function we check if the passed arguments count and type matches the function signature. We should also check if the fn_name matches the function name part in the fn_signature and raise and error if they don't match.

https://thegraph.com/docs/en/developer/matchstick/#mocking-contract-calls https://github.com/LimeChain/matchstick/blob/main/src/context.rs#L818

Estimate: 1 day