[electrum] use the docker-compose command for functional tests

Summary: As of pytest-docker 2.0.0, docker-compose is no longer the default command. The default docker compose command now requires a newer version of docker than what is packaged for debian, so add a docker_compose_command pytest fixture to switch back to the previous behavior.

Test Plan: @bot electrum-functional-tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14291