chore: do not rely on src libraries in test libraries

[fedgiac]

Description

We define a broad set of library functions to make testing the contracts in src easier. However, these libraries themselves rely on the tested contracts. This could lead to issues when the tests pass because both the libraries and the files in src are broken in the same way. To guarantee that tests are independent of the tested libraries, all use of src libraries has been removed.

What I really wanted

Unfortunately, it's still possible that we forget about this and use some function in, e.g., GPv2Order in the test library, especially since we need to import this library to get related types and constants.

I tried to create a Solidity file that reexports everything we need from the libraries without the functions themselves (this would, for example, allow us to define a custom linting rule asserting that there's no import from src in any test file) but I was unsuccessful. It was very easy to reexport contracts (e.g., GPv2Settlement) and interfaces (e.g., IERC20), and it was also easy to reexport constants (e.g., GPv2Order.KIND_SELL), The issue was with reexporting type structs: to my understanding, this is impossible in Solidity ≤0.8.26, confirmed by the fact that there is no statement that appears to enable this use case in the language grammar.
Since neither reexporting everything but the structs nor reexporting the entire libraries make sense, I decided to keep everything as it is without further reexporting.

Test Plan

Nothing broken in CI.