By Debian stretch default, there are some default systemd user unit
files in `/usr/lib/systemd/user/`.
With a different path on Debian stretch
`/usr/lib/systemd/user/mytest.service`:
```
[Unit]
Description=mytest
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/true
[Install]
WantedBy=default.target
```
systemctl --user enable mytest
Worked for me.
> `Created symlink
/home/user/.config/systemd/user/default.target.wants/mytest.service →
/usr/lib/systemd/user/mytest.service.`
After reboot, mytest.service will be active.
