MockBuilder: add support for traits with methods with the same name

[lemunozm]

Description

Add support for testing pallets that implement traits whose methods have the same name.

When a pallet, implements two traits with the same methods, the trait name should now be in the name signature (only needed in case of name collision, otherwise the pallet can be constructed as always). Example:

struct Pallet;

impl Pallet {
    #[allow(non_snake_case)]
    pub fn mock_TraitA_get(f: impl Fn(u64) -> usize + 'static) {
        register_call!(f);
    }

    #[allow(non_snake_case)]
    pub fn mock_TraitB_get(f: impl Fn(i32) -> bool + 'static) {
        register_call!(f);
    }
}

impl TraitA for Pallet {
    fn get(a: u64) -> usize {
        execute_call!(a)
    }
    
    fn get(a: i32) -> bool {
        execute_call!(a)
    }
}

This change is retrocompatible

Fixes centrifuge/centrifuge-chain#1564

[wischli]

Great addition to the mock-builder operationality! Thanks for picking this up. Three non-blocking super minor nits:

[lemunozm]

Thanks for your fast review!

[lemunozm]

This project has no CI yet, but it has passed all tests.