tst: fix reverse_dns test

[wagner-intevation]

remove test_invalid_ptr as the IP address used no longer returns the
expected result

fixes #2206

[codecov-commenter]

Codecov Report

Merging #2208 (7969687) into develop (79cae29) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2208      +/-   ##
===========================================
- Coverage    76.34%   76.22%   -0.13%     
===========================================
  Files          441      441              
  Lines        23654    23632      -22     
  Branches      3739     3437     -302     
===========================================
- Hits         18059    18013      -46     
- Misses        4858     4880      +22     
- Partials       737      739       +2     

Impacted Files	Coverage Δ
...elmq/tests/bots/experts/reverse_dns/test_expert.py	100.00% <ø> (ø)
intelmq/bots/experts/reverse_dns/expert.py	83.87% <0.00%> (-11.30%)	⬇️
intelmq/bots/experts/mcafee/expert_mar.py	35.55% <0.00%> (-8.89%)	⬇️
intelmq/bots/experts/modify/expert.py	92.00% <0.00%> (-5.34%)	⬇️
intelmq/bots/parsers/dshield/parser_domain.py	95.45% <0.00%> (-4.55%)	⬇️
intelmq/tests/lib/test_exceptions.py	95.65% <0.00%> (-4.35%)	⬇️
intelmq/bots/collectors/opendxl/collector.py	30.15% <0.00%> (-3.18%)	⬇️
intelmq/bots/parsers/dshield/parser_block.py	87.50% <0.00%> (-2.50%)	⬇️
intelmq/bots/collectors/tcp/collector.py	84.31% <0.00%> (-1.97%)	⬇️
intelmq/bots/parsers/generic/parser_csv.py	85.18% <0.00%> (-1.86%)	⬇️
... and 12 more