pkg/karmadactl/util: unit test apiclient

[mohamedawnallah]

Description

In this commit, we unit test the apiclient to verify the creation of a restconfig in both scenarios: when it exists and when it doesn't.

What type of PR is this?

Which issue(s) this PR fixes:
Part of #5491.

Does this PR introduce a user-facing change?:

NONE

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 46.24%. Comparing base (88c3e44) to head (ddfad3f).

Files with missing lines	Patch %	Lines
pkg/karmadactl/util/apiclient/apiclient.go	50.00%	2 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5869      +/-   ##
==========================================
+ Coverage   46.20%   46.24%   +0.03%     
==========================================
  Files         663      663              
  Lines       54580    54578       -2     
==========================================
+ Hits        25218    25237      +19     
+ Misses      27739    27714      -25     
- Partials     1623     1627       +4     

Flag	Coverage Δ
unittests	46.24% <50.00%> (+0.03%)	⬆️

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