Added tests for mcs controller

[anujagrawal699]

Description: This PR introduces comprehensive unit tests for the Multi-Cluster Services (MCS) controllers. These tests aim to improve code reliability and facilitate easier maintenance and future development.

Additions:

1. pkg/controllers/mcs/endpointslice_controller_test.go
2. pkg/controllers/mcs/service_export_controller_test.go
3. pkg/controllers/mcs/service_import_controller_test.go

Test Coverage:

1. endpointslice_controller_test.go : 0% to 46%
2. service_export_controller_test.go : 0% to 39.1%
3. service_import_controller_test.go : 0% to 65%

Overall coverage of MCS controller : 0% to 44.4%

What type of PR is this? /kind failing-test /kind feature

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

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 34.43%. Comparing base (ec0521a) to head (8a4923f). Report is 346 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5566      +/-   ##
==========================================
+ Coverage   34.15%   34.43%   +0.27%     
==========================================
  Files         643      645       +2     
  Lines       44515    44857     +342     
==========================================
+ Hits        15203    15445     +242     
- Misses      28155    28240      +85     
- Partials     1157     1172      +15     

Flag	Coverage Δ
unittests	34.43% <ø> (+0.27%)	:arrow_up:

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🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[XiShanYongYe-Chang]

Thanks /assign

[karmada-bot]

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[anujagrawal699]

@XiShanYongYe-Chang Made the changes as you suggested.