service: Simplify and test MeshService.FQDN()

[draychev]

This PR is an attempt to improve the readability of FQDN(). But most importantly - test it and show what it produces.

Signed-off-by: Delyan Raychev [email protected]

[codecov-commenter]

Codecov Report

Merging #3826 (8bbbe98) into main (94394bd) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3826      +/-   ##
==========================================
+ Coverage   67.38%   67.58%   +0.19%     
==========================================
  Files         198      198              
  Lines       10978    10978              
==========================================
+ Hits         7398     7419      +21     
+ Misses       3531     3510      -21     
  Partials       49       49              

Flag	Coverage Δ
unittests	67.58% <100.00%> (+0.19%)	⬆️

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Impacted Files	Coverage Δ
pkg/service/types.go	50.00% <100.00%> (+21.42%)	⬆️
...icate/providers/certmanager/certificate_manager.go	86.61% <0.00%> (+0.70%)	⬆️
pkg/crdconversion/crdconversion.go	59.78% <0.00%> (+18.47%)	⬆️

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