This test requires `electrs` to be enabled.
Also, by using `electrs` instead of `clightning` we avoid the bug
where clightning hangs at startup when internet access is unavailable.
Due to technical limititations, internet access is available in NixOS
containers only after the container has started.
Some of the commands listed in usage.sh require clightning to be
enabled. This commit enables clightning so they will work. It also
removes electrs to keep things simple, since electrs is not in
usage.sh