Add a final empty label (.) on DNS strings

[p-l-]

The added test should make the reason of this change clear: for now, the .qname returned values differ between crafted and captured packets.

Now the .qname attribute always shows the final empty label (or final ".").

Reported by @Frky

[codecov[bot]]

Codecov Report

Merging #3710 (b4e2689) into master (08b1f9d) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3710      +/-   ##
==========================================
- Coverage   86.11%   86.11%   -0.01%     
==========================================
  Files         296      296              
  Lines       67286    67290       +4     
==========================================
+ Hits        57942    57945       +3     
- Misses       9344     9345       +1     

Impacted Files	Coverage Δ
scapy/layers/dns.py	91.74% <100.00%> (+0.06%)	:arrow_up:
scapy/arch/windows/__init__.py	67.73% <0.00%> (-0.57%)	:arrow_down:
scapy/contrib/automotive/ecu.py	94.33% <0.00%> (-0.34%)	:arrow_down:
scapy/pipetool.py	84.10% <0.00%> (-0.23%)	:arrow_down:
scapy/compat.py	92.81% <0.00%> (+0.59%)	:arrow_up:
scapy/layers/l2.py	76.71% <0.00%> (+0.68%)	:arrow_up:

[p-l-]

Thanks for that @gpotter2!

[p-l-]

rebased after #3711