Restructure Almanac tests in uAgents

[Archento]

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Current situation:

• agent_registration.py has 4 tests (2 sync, 2 async)
• the tests in general take around 6 seconds each, most of the times ~20 seconds
• since we apply a GitHub Actions matrix these tests will be executed 7 times on different systems/versions
• sometimes they fail due to issues with the ledger/network or timeouts
• these tests take a lot of time / costing a lot of money compared to the rest of the CI workflow

We should rethink how to effectively test the registration functionality as that is not completely part of the uAgents framework but rather includes or represents an interface to the Almanac. Maybe we can mock the smart contract responses or just take these test to a separate worker.