tests: Use FQDNs as bad nodes hostnames

[fihuer]

In tests, use FQDNs for node that are expected to be (not) resolved as NXDOMAIN.

Without this, in environnements where local domain have wildcards DNS, anything (except FQDNs) can be resolved to something.

[codecov-commenter]

Codecov Report

Merging #53 (e1f8875) into master (16d4ee0) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   99.55%   99.53%   -0.03%     
==========================================
  Files          22       22              
  Lines        4984     4984              
==========================================
- Hits         4962     4961       -1     
- Misses         22       23       +1     

Impacted Files	Coverage Δ
tests/MilkCheckTests/EngineTests/ActionTest.py	100.00% <100.00%> (ø)
...sts/MilkCheckTests/EngineTests/ServiceGroupTest.py	100.00% <100.00%> (ø)
tests/MilkCheckTests/EngineTests/ServiceTest.py	100.00% <100.00%> (ø)
tests/MilkCheckTests/UITests/CliTest.py	99.78% <100.00%> (-0.22%)	⬇️

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