test: make TEST-07-PID1.user-namespace-path more robust (#42001)

The test occasionally fails because lsns returns empty output for the
transient unit, even though the process is running. e.g.:

```
    [ 1843.556046] TEST-07-PID1.sh[8560]: + systemd-run --unit=newservice --property=Type=exec --property=UserNamespacePath=/proc/8608/ns/user --property=NetworkNamespacePath=/proc/8608/ns/net sleep 3600
    [ 1844.205927] TEST-07-PID1.sh[8616]: ++ systemctl show newservice -p MainPID
    [ 1844.221425] TEST-07-PID1.sh[8618]: ++ lsns -p 8608 -o NS -t net -n
    [ 1844.229653] TEST-07-PID1.sh[8619]: ++ lsns -p 8614 -o NS -t net -n
    [ 1844.235563] TEST-07-PID1.sh[8620]: FAIL: expected: '' actual: '4026532522'
```

This could be a race, so switch to Type=notify to try and make it more
robust.

eg:
https://github.com/systemd/systemd/actions/runs/25485775345/job/74780837128?pr=41974