nixos/tests/netdata: fix non-deterministic failure

The test sporadically failed on hydra when a request was made
before the service was actually listening on its port.
Explicitly wait for the port to open.

nixos/tests/netdata.nix

···
 
     $netdata->waitForUnit("netdata.service");
 
+    # wait for the service to listen before sending a request
+    $netdata->waitForOpenPort(19999);
+
     # check if the netdata main page loads.
     $netdata->succeed("curl --fail http://localhost:19999/");