Modularize integration test framework

[justinmoon]

• The test framework defined inside integrationtests should probably become a library that could be consumed by external module developers. Perhaps it could live in the same crate as "(1)" defined in this issue about splitting fedimint-bin-tests.
• The test framework would need to be modularized. AFAIK there is currently no way to add or remove modules.

[jkitman]

Thinking about this we need to go through the following:

• [x] Remove the old FakeFed making sure we already have enough unit/integration test coverage
• [x] Make fedimint-testing module-agnostic
• [x] Refactor BTC/LN fixtures from integrationtests into fedimint-testing
• [x] Create new federation fixtures in fedimint-testing against dummy module
• [x] Write modular mint tests with new client
• [x] Write modular lightning tests with new client
• [x] Write modular wallet tests with new client
• [ ] Test all "expected" client state transitions
• [ ] Test some malicious clients/peer conditions
• [ ] Remove legacy client and test framework

Ideal features of the new test framework:

• Modular and supports adding the dummy module for fast printing of assets/liabilities
• Will work after switch to AlephBFT consensus (and consensus should be mockable)
• Supports mocking the client-server network layer
• Simulate federation nodes restarting (test more of the actual server code)