test: ensure cleanup functions return success

test: ensure cleanup functions return success

Otherwise the return value of the last command is propagated, which may
cause spurious test failures. E.g., pkill returns 1 if no process
matched, which may be a problem in cleanup session:

cleanup_session() {
...

    pkill -u "$(id -u logind-test-user)"
    sleep 1
    pkill -KILL -u "$(id -u logind-test-user)"
}

If there are no remaining processes when the final pkill runs, it will
return 1 and therefore cleanup_session will return 1 as well.