Add tests based on Ampleforth Geyser v2 tests

[itirabasso]

adapted from https://github.com/ampleforth/token-geyser-v2

• update references from Geysey to Aludel, UniversalVault to Crucible, Ampl to Mist
• remove rebase mechanism tests
• add mock contracts
• use the shared-before-each plugin to improve tests timing, from 2m40s to 55s

coverage is at 80

Still missing tests for: steamV1, streamV2, Transmuter, TokenManager, and some getters from the Alchemist contract.