dns: stop recording unknown names in the answering machine

[KernelClint]

DNS_am answers queries from a match mapping of name to address, falling back to the documented
joker and joker6 values for anything not configured.

That mapping is a defaultdict built at
scapy/layers/dns.py:1677,
and the A and AAAA paths reach the fallback by indexing it (:1811, :1827). Indexing a
defaultdict with a missing key inserts it. Every query for a name the operator never configured
therefore adds an entry, and the mapping grows for as long as the machine runs, driven entirely by
what arrives. LLMNR_am inherits the same behaviour.

The change uses a plain dictionary and handles the missing key without writing to it:

-        self.match = collections.defaultdict(lambda: (joker, joker6))
+        self.match = {}
...
-                    rdata = self.match[rqname][0]
+                    try:
+                        rdata = self.match[rqname][0]
+                    except KeyError:
+                        rdata = self.joker

Configured names resolve as before and both jokers keep their documented behaviour; the only change
is that an unknown name is no longer recorded.

The added regressions cover A and AAAA and assert the mapping is still empty after answering a
series of unknown names. Without the source change they fail.

Performance was measured on one computer, before and after the fix: answering a configured name
took 84.5 ns before and 86.8 ns after, a difference of 2.3 ns. Repeat runs moved by about 2%, which
is the same size as the difference, so the test cannot separate them.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (b3bbcc8) to head (96530b9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5136   +/-   ##
=======================================
  Coverage   80.63%   80.64%           
=======================================
  Files         390      390           
  Lines       96936    96941    +5     
=======================================
+ Hits        78168    78177    +9     
+ Misses      18768    18764    -4     

Files with missing lines	Coverage Δ
scapy/layers/dns.py	83.95% <100.00%> (+0.09%)	⬆️

... and 3 files with indirect coverage changes

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