Make psconfig building ESnet matrix for DNS query test when DNS server is specified

[szymontrocha]

I have the following use case: I want to have DNS request tests visualised with the specific DNS server specified in the test. So I built the following JSON test spec:

    "test_dns_sunflower": {
      "type": "dns",
      "spec": {
        "record": "a",
        "query": "{% address[1] %}",
        "host": "{% address[0] %}",
        "nameserver": "150.254.173.2"
      }
    "test_dns_rose": {
      "type": "dns",
      "spec": {
        "record": "a",
        "query": "{% address[1] %}",
        "host": "{% address[0] %}",
        "nameserver": "150.254.173.3"
      }
    },

However this results in improper Grafana dashboard containing empty matrixes:

Also reference from the matrix are not correct: