Delete local `InvariantTest` util and use `StdInvariant` from `forge-std`

[Paul Razvan Berg]

Forge Std now comes with a StdInvariant that offers the same functionality as the local InvariantTest file from test/utils:

https://github.com/foundry-rs/forge-std/blob/7f7cb06aa55fc75d2280d7d54713dc4368d71fd3/src/StdInvariant.sol

[Paul Razvan Berg]

Closed by #215.