netconf-connector readiness test suite checking non existent resource

netconf-connector readiness suite mounts CSS subsystem netconf endpoint and checks if mounting succeeded. This check is done by trying to access resources on CSS netconf server through newly created mountpoint. We can see this for example in this job https://logs.opendaylight.org/releng/jenkins092/netconf-csit-1node-periodic-scale-all-nitrogen/116/log.html.gz#s1-s1-s1-t1-k3-k1-k1-k1-k1-k5-k1-k2-k1-k1-k3.

The problem is that we are trying to access restconf/config/network-topology:network-topology/topology/topology-netconf/node/test-device/yang-ext:mount/config:modules/module/sal-restconf-service:json-restconf-service-impl/json-restconf-service-impl resource, which is no longer present in the system. This was removed in https://git.opendaylight.org/gerrit/#/c/59633/

I suggest we should check for example netconf-state to see if we can access resources on mounted CSS through mountpoint and be sure that netconf is ready for following suites.