Add required tests for go/netutil

[beingnoble03]

Description

Adds required tests for go/netutil

Related Issue(s)

• Fixes part of Improve Unit Test Coverage #14931

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[beingnoble03]

I don't think it would be helpful to add test for FullyQualifiedHostname. Should we add test for it?

[ajm188]

LGTM with the one assertion change

[ajm188]

outside the scope of this PR, but should we delete this function in favor of the stdlib net.JoinHostPort? cc @mattlord, the signature is slightly different (we take an int32 port whereas stdlib takes string)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.46%. Comparing base (696fe0e) to head (fda1a9b).
Report is 71 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15392      +/-   ##
==========================================
- Coverage   67.41%   65.46%   -1.96%     
==========================================
  Files        1560     1562       +2     
  Lines      192752   193925    +1173     
==========================================
- Hits       129952   126959    -2993     
- Misses      62800    66966    +4166     

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