test: Unify systemctl calls
test: Unify systemctl calls

There were a few instances where we called `systemctl --user foo` but we
ran this as root. I have extracted the logic into `run_systemctl()` that
does the correct thing both for user and system sessions.

mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit