tests: install execs from user units too

Fixes:
systemd[39]: systemd-exit.service: Executing: /bin/kill -s 58 29
systemd[39]: systemd-exit.service: Failed at step EXEC spawning /bin/kill: No such file or directory
systemd[29]: Received SIGCHLD from PID 39 ((kill)).
systemd[29]: Child 39 ((kill)) died (code=exited, status=203/EXEC)