chore(PocketIC): retry in PocketIC bitcoin integration tests

[mraszyk]

This PR prevents spurious errors in PocketIC bitcoin integration tests

---- bitcoin_integration_test stdout ----
thread 'bitcoin_integration_test' panicked at rs/pocket_ic_server/tests/bitcoin_integration_tests.rs:145:10:
called `Result::unwrap()` on an `Err` value: JsonRpc(Transport(SocketError(Os { code: 111, kind: ConnectionRefused, message: "Connection refused" })))
stack backtrace:

by retrying generating blocks until the bitcoind is up and running.