resolver: add some additional test coverage

[gdamore]

While here, remove some code paths that do not occur by definition. (For example, if the resolver succeeds, we will definitely have a valid set of addresses, but if it fails, we will definitely not.)

fixes #

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[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.69%. Comparing base (a1bc59a) to head (b5462f6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/platform/posix/posix_resolv_gai.c	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2030      +/-   ##
==========================================
+ Coverage   82.55%   82.69%   +0.13%     
==========================================
  Files          95       95              
  Lines       23662    23656       -6     
  Branches     3046     3044       -2     
==========================================
+ Hits        19534    19562      +28     
+ Misses       4058     4023      -35     
- Partials       70       71       +1     

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