[marlin] Prepare for end to end send tests

Summary:
This diff mostly update the fixtures and tooling in preparation for adding send tests:

• It adds the required elements to the stubs, aka the utxos and blockchain info
• it adds an interceptor for the txs broadcast (only successful for now)
• It lets the visitWithWalletMnemonic initializer set options (so we can avoid the default "hold to send" in tests)
• it adds a simple send test to demonstrate all this machinery

Test Plan:

pnpm test:e2e

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D19819