Added unit tests for the scheduler's node tree functionality

[anujagrawal699]

Description:
This PR adds comprehensive unit tests for the scheduler's node tree functionality. These tests improve code coverage and ensure reliable behavior of a critical component that manages node organization by zones for efficient scheduling decisions.

Additions:

1. pkg/util/lifted/scheduler/cache/node_tree_test.go

Test Coverage:

1. pkg/util/lifted/scheduler/cache/node_tree_test.go : 0% to 98.2%

What type of PR is this?
/kind feature

Which issue(s) this PR fixes:
Fixes a part of #5470

Does this PR introduce a user-facing change?:

NONE

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.10%. Comparing base (057cf86) to head (da08542).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5772      +/-   ##
==========================================
- Coverage   42.31%   42.10%   -0.21%     
==========================================
  Files         655      655              
  Lines       55756    55756              
==========================================
- Hits        23591    23477     -114     
- Misses      30651    30789     +138     
+ Partials     1514     1490      -24     

Flag	Coverage Δ
unittests	42.10% <ø> (-0.21%)	⬇️

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[zhzhuang-zju]

[CronFederatedHPA] CronFederatedHPA testing Scale FederatedHPA [It] Test scale FederatedHPA testing
https://github.com/karmada-io/karmada/actions/runs/11640040601/job/32417012568?pr=5772

/retest