enhance top pod command

[hulizhe]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

• Improve the example for the top pod command.
• Combine multiple ginkgo.It tests into one.

Which issue(s) this PR fixes:
Parts of #5249

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 31.15%. Comparing base (c9e0ad7) to head (1a34a70).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5452      +/-   ##
==========================================
- Coverage   31.15%   31.15%   -0.01%     
==========================================
  Files         640      640              
  Lines       44417    44417              
==========================================
- Hits        13837    13836       -1     
  Misses      29580    29580              
- Partials     1000     1001       +1     

Flag	Coverage Δ
unittests	31.15% <ø> (-0.01%)	⬇️

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[RainbowMango]

/lgtm
/approve

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

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Needs approval from an approver in each of these files:

• pkg/karmadactl/top/OWNERS [RainbowMango]
• test/OWNERS [RainbowMango]

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