Support multiple TXT entries for TLD

The dig of TXT for @ can returns multiple entries.
In that case, the DNS diagnosis fails.

The modification preserves the handling of DMARC and the likes
which use a single entry and a specfic domain name.

For single entry list, the behavior is preserved.

If mutliple TXT entries are defined for @, only the v=spf1
one is returned.

Signed-off-by: Nicolas Palix <nicolas.palix@imag.fr>

src/diagnosers/12-dnsrecords.py

@@ -182,6 +182,10 @@ class MyDiagnoser(Diagnoser):
         if success != "ok":
             return None
         else:
+            if type_ == "TXT" and isinstance(answers,list):
+                for part in answers:
+                    if part.startswith('"v=spf1'):
+                        return part
             return answers[0] if len(answers) == 1 else answers
 
     def current_record_match_expected(self, r):