Added unit tests for node_tree in pkg/util/lifted/scheduler/cache

[yashpandey06]

/kind feature

This PR introduces unit tests for the node_tree file :

1. TestNewNodeTree: Initialize a NodeTree with nodes.
2. TestAddNode: Add a new node to the NodeTree.
3. TestAddExistingNode: Handle adding an existing node.
4. TestRemoveNode: Remove a node from the NodeTree.
5. TestRemoveNonExistentNode: Attempt to remove a non-existent node.
6. TestUpdateNode: Update node details.
7. TestList: List all nodes.
8. TestListEmpty: Handle an empty NodeTree.
9. TestListExhaustedZones: Handle exhausted zones.

Fixes part of #5235

Coverage : 96.5 %

Does this PR introduce a user-facing change?:

NONE

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 30.40%. Comparing base (aee21b8) to head (e21572d). Report is 24 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5422      +/-   ##
==========================================
- Coverage   30.58%   30.40%   -0.18%     
==========================================
  Files         632      632              
  Lines       43936    43936              
==========================================
- Hits        13438    13360      -78     
- Misses      29519    29622     +103     
+ Partials      979      954      -25     

Flag	Coverage Δ
unittests	30.40% <ø> (-0.18%)	:arrow_down:

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[XiShanYongYe-Chang]

/retest

[yashpandey06]

@XiShanYongYe-Chang , the pr has passed the ci tests, we can move ahead with the review .

Thanks 😊