Dockerize Electrum servers and migrate tests from remote Electrums

[shamardy]

Add Electrum server containers (ElectrumX) to .docker/test-nodes.yml to enable Electrum-based testing.

This builds on the docker-compose infrastructure from #2707. Electrum tests can share CI jobs with existing UTXO tests since they use the same blockchain nodes.

Supersedes #2602 once implemented.

[shamardy]

Tests ignored pending Electrum dockerization

The following tests have been temporarily disabled with #[ignore] and a comment pointing to this issue:

Unit Tests (coins crate)

• test_unavailable_electrum_proto_version - utxo/utxo_tests.rs
• test_one_unavailable_electrum_proto_version - utxo/utxo_tests.rs
• test_qtum_add_delegation - utxo/utxo_tests.rs
• test_qtum_get_delegation_infos - utxo/utxo_tests.rs
• test_wait_for_confirmations_excepted - qrc20/qrc20_tests.rs

Integration Tests (mm2_main)

• test_electrum_tx_history - mm2_tests_inner.rs
• test_add_delegation_qtum - mm2_tests_inner.rs
• test_query_delegations_info_qtum - mm2_tests_inner.rs
• test_qrc20_withdraw - mm2_tests_inner.rs

All these tests rely on remote Electrum servers (cipig.net, etc.) which have intermittent availability issues. Once ElectrumX containers are added to the docker-compose infrastructure, these tests should be re-enabled with local container connections.

See PR #2707 for the changes.