Generalizing erc4626 buffer tests

[elshan-eth]

Description

The initialization of erc4626 buffer tests was repeated in different tests. This PR adds a generalizing erc4626 contract for using common initialization.

Type of change

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Dependency changes
• [ ] Code refactor / cleanup
• [ ] Optimization: [ ] gas / [ ] bytecode
• [ ] Documentation or wording changes
• [X] Other

Checklist:

• [ ] The diff is legible and has no extraneous changes
• [ ] Complex code has been commented, including external interfaces
• [ ] Tests have 100% code coverage
• [ ] The base branch is either main, or there's a description of how to merge

Issue Resolution

Closes #808

[EndymionJkb]

Nice, looks good to me. I'm still confused if we're gonna call the pool that holds yield-bearing tokens a yield-bearing pools, boosted pool or erc4626 pool. I think we should decide on that to keep the standard. @EndymionJkb @jubeira , which one should we adopt (or have we adopted yield-bearing pool already)?

We don't want "marketing" terms in the code, so definitely not "boosted" pool. We have been using "erc4626" for the tokens (as this is explicit in the code), and "yield-bearing" for the pool, as ERC4626 tokens are yield-bearing by definition, so I'd stick with that.