test: adjust test-path to fail gracefully with the new pidfd_spawn stuff

test: adjust test-path to fail gracefully with the new pidfd_spawn stuff

Since 2e106312e2 the test unit fails with 'resources' result instead of
'exit-code', which the test didn't account for when running unprivileged.

Before 2e106312e2:
$ /root/systemd/build/test-path
Failed to start transient scope unit: Interactive authentication required.
Couldn't allocate a scope unit for this test, proceeding without.
...
-.slice: Failed to enable/disable controllers on cgroup /user.slice/user-1000.slice/session-1.scope, ignoring: Permission denied
app.slice: Failed to create cgroup /user.slice/user-1000.slice/session-1.scope/app.slice: Permission denied
-.slice: Failed to enable/disable controllers on cgroup /user.slice/user-1000.slice/session-1.scope, ignoring: Permission denied
app.slice: Failed to create cgroup /user.slice/user-1000.slice/session-1.scope/app.slice: Permission denied
...
line 151: path-exists.path: state = running; result = success (left: 29986250)
line 151: path-exists.service: state = start; result = success
path-exists.service: Main process exited, code=exited, status=219/CGROUP
path-exists.service: Failed with result 'exit-code'.
line 151: path-exists.path: state = running; result = success (left: 29985948)
line 151: path-exists.service: state = failed; result = exit-code
Failed to start service path-exists.service, aborting test: failed/exit-code

After 2e106312e2:
$ /root/systemd/build/test-path
Failed to start transient scope unit: Interactive authentication required.
Couldn't allocate a scope unit for this test, proceeding without.
...
-.slice: Failed to enable/disable controllers on cgroup /user.slice/user-1000.slice/session-1.scope, ignoring: Permission denied
app.slice: Failed to create cgroup /user.slice/user-1000.slice/session-1.scope/app.slice: Permission denied
-.slice: Failed to enable/disable controllers on cgroup /user.slice/user-1000.slice/session-1.scope, ignoring: Permission denied
app.slice: Failed to create cgroup /user.slice/user-1000.slice/session-1.scope/app.slice: Permission denied
path-exists.service: Failed to spawn executor: No such file or directory
path-exists.service: Failed to spawn 'start' task: No such file or directory
path-exists.service: Failed with result 'resources'.