test: consolidate checks for unpriv nspawn support in TEST-13-NSPAWN

test: consolidate checks for unpriv nspawn support in TEST-13-NSPAWN

[   69.058386] systemd-nspawn[4371]: varlink: Sending message: {"method":"io.systemd.NamespaceResource.AllocateUserRange","parameters":{"name":"nspawn-4371-zurps","mangleName":true,"size":65536,"userNamespaceFileDescriptor":0}}
[   69.058447] systemd-nsresourcework[4339]: varlink-6-6: Received message: {"method":"io.systemd.NamespaceResource.AllocateUserRange","parameters":{"name":"nspawn-4371-zurps","mangleName":true,"size":65536,"userNamespaceFileDescriptor":0}}
[   69.058455] systemd-nsresourcework[4339]: varlink-6-6: Changing state idle-server → processing-method
[   69.058479] systemd-nsresourcework[4339]: varlink-6-6: Sending message: {"error":"io.systemd.NamespaceResource.UserNamespaceInterfaceNotSupported","parameters":{}}
[   69.058482] systemd-nsresourcework[4339]: varlink-6-6: Changing state processing-method → processed-method
[   69.058486] systemd-nsresourcework[4339]: varlink-6-6: Changing state processed-method → idle-server
[   69.058599] systemd-nspawn[4371]: varlink: Received message: {"error":"io.systemd.NamespaceResource.UserNamespaceInterfaceNotSupported","parameters":{}}
[   69.058604] systemd-nspawn[4371]: varlink: Changing state calling → called
[   69.058609] systemd-nspawn[4371]: varlink: Changing state called → idle-client
[   69.058614] systemd-nspawn[4371]: Unprivileged user namespace delegation is not supported on this system.
[   69.058637] systemd-nsresourcework[4339]: varlink-6-6: Got POLLHUP from socket.
[   69.058647] systemd-nsresourcework[4339]: varlink-6-6: Changing state idle-server → pending-disconnect
[   69.058653] systemd-nsresourcework[4339]: varlink-6-6: Changing state pending-disconnect → processing-disconnect
[   69.058656] systemd-nsresourcework[4339]: varlink-6-6: Changing state processing-disconnect → disconnected
[   69.058698] systemd-nspawn[4371]: Failed to allocate user namespace with 64K users: Operation not supported
[   69.058779] systemd[4344]: systemd-nspawn@zurps.service: Got notification message from PID 4371: STOPPING=1, STATUS=Terminating...

Follow-up for bfd356da63d9fe0720f1b5a61c527c8822c3b808