Enable fuzzing in fork tests via deterministic seeds

[davidlaprade]

Most of our Aave-related tests are fork tests. For this reason, we don't fuzz over their inputs -- as this would generate a massive (and expensive) number of RPC calls. But it's possible to fuzz using a deterministic seed, which would allow us to cache RPC calls and avoid unnecessary network interactions.

Two tasks here:

• support deterministic fuzzing in CI
• convert existing Aave-related fork tests into fuzz tests

RE: ScopeLift/flexible-voting#21 (comment)
Copied from ScopeLift/flexible-voting#25