Suppress warning/error comments in dig outputs for "rrsetorder" test

Suppress warning/error comments in dig outputs for "rrsetorder" test

In the CI dig sometimes produces warning/error comments when
communicating with the server, which produces problems when comparing
the outputs.

Here is an example of a dig output with a warning message which
is benign, because dig, after a retry, managed to query the server.

;; communications error to 10.53.0.3#7529: timed out
1.2.3.1
1.2.3.2
1.2.3.3
1.2.3.4

When comparing this to the expected output, which doesn't contain
the comment line (starting with double ';'), the outputs don't match.

Use grep inverse logic to strip the comments from the dig outputs.

(cherry picked from commit a5b2aa7c977b5c40b62698a8f46ffd839b764814)