test: wait until the nspawn process is completely dead (#39576)

test: wait until the nspawn process is completely dead  (#39576)

Before calling io.systemd.MachineImage.List.

The systemd-nspawn process takes a lock in the run() function in
nspawn.c and holds it for the entire runtime of that function. If we
call `machinectl terminate` the machine gets unregistered _before_ we
release the lock, so the original `machinectl status` check would return
early, allowing for a race where we call io.systemd.MachineImage.List
over Varlink when systemd-nspawn still holds the lock because the
process is still running.:

```
[   41.691826] TEST-13-NSPAWN.sh[1102]: + machinectl terminate long-running
[   41.695009] systemd-nspawn[2171]: Trying to halt container by sending TERM to container PID 1. Send SIGTERM again to trigger immediate termination.
[   41.698235] systemd-machined[1192]: Machine long-running terminated.
[   41.709520] TEST-13-NSPAWN.sh[1102]: + systemctl kill --signal=KILL systemd-nspawn@long-running.service
[   41.709169] systemd-nspawn[2171]: Failed to unregister machine: No machine 'long-running' known
[   41.720869] TEST-13-NSPAWN.sh[2346]: + varlinkctl --more call /run/systemd/machine/io.systemd.MachineImage io.systemd.MachineImage.List '{}'
[   41.723359] TEST-13-NSPAWN.sh[2347]: + grep long-running
...
[   41.735453] TEST-13-NSPAWN.sh[2352]: + varlinkctl call /run/systemd/machine/io.systemd.MachineImage io.systemd.MachineImage.List '{"name":"long-running", "acquireMetadata": "yes"}'
[   41.736222] TEST-13-NSPAWN.sh[2353]: + grep OSRelease
[   41.739500] TEST-13-NSPAWN.sh[2352]: Method call io.systemd.MachineImage.List() failed: Device or resource busy
[   41.740641] systemd[1]: Received SIGCHLD.
[   41.740670] systemd[1]: Child 2171 (systemd-nspawn) died (code=killed, status=9/KILL)
[   41.740725] systemd[1]: systemd-nspawn@long-running.service: Child 2171 belongs to systemd-nspawn@long-running.service.
[   41.740748] systemd[1]: systemd-nspawn@long-running.service: Main process exited, code=killed, status=9/KILL
[   41.740755] systemd[1]: systemd-nspawn@long-running.service: Will spawn child (service_enter_stop_post): systemd-nspawn
[   41.740872] systemd[1]: systemd-nspawn@long-running.service: About to execute: systemd-nspawn --cleanup --machine=long-running
...
```

Let's mitigate this by waiting until the corresponding
systemd-nspawn@.service instance enters the 'inactive' state where the
lock should be properly released.

Resolves: https://github.com/systemd/systemd/issues/39547