fix(Justfile): fix test-threads to max 2

fix(Justfile): fix test-threads to max 2

- The problem: When running `cargo test --all-
features`, the backends (electrum, rpc, esplora
from TestEnv) run in parallel at the number of
cpus available and exhaust the processes/ports
available, thereby resulting in os error 35.

This fix limits the number of threads that are
available for the tests to run in parallel.