Better testing strategy

[beltram]

The goal is to have better tests, better coverage but first, tests should be easy to write and maintain

Acceptance criterias:

• provide helpers to write tests on conversations (MlsConversation)
• Move all test fixtures in a dedicated crate (crypto-fixtures ?)
• All tests involving a conversation must be tested both on MlsCentral and MlsConversation
• All tests must be executed in WASM