SearXNG quadlet: use AddCapability= (the generator rejects CapAdd=)

The podman quadlet generator fails with "unsupported key 'CapAdd' in group
'Container'" and skips the whole unit. The correct key is AddCapability=
(maps to podman --cap-add). Update the gluetun unit and its test seam.

dankclaw/searxng/quadlet/gluetun.container

@@ -17,7 +17,7 @@ After=searxng-network.service network.target
 ContainerName=gluetun
 Network=searxng.network
 Image=docker.io/qmcgaw/gluetun:latest
-CapAdd=NET_ADMIN
+AddCapability=NET_ADMIN
 Device=/dev/net/tun:/dev/net/tun
 Environment=VPN_SERVICE_PROVIDER=${VPN_SERVICE_PROVIDER}
 Environment=VPN_TYPE=${VPN_TYPE}