pkg/search/backendstore: unit test store

[mohamedawnallah]

Description

In this commit, we unit test backend store in karmada search on init backendstore manager, add backendstore, delete backendstore, and on get backendstore operations in addition to testing the concurrent calls on those ops.

What type of PR is this?

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

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

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.60%. Comparing base (3acc14c) to head (6ae64b2).
Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/search/backendstore/opensearch.go	33.33%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5811      +/-   ##
==========================================
+ Coverage   45.56%   45.60%   +0.04%     
==========================================
  Files         658      658              
  Lines       53921    53923       +2     
==========================================
+ Hits        24567    24590      +23     
+ Misses      27750    27721      -29     
- Partials     1604     1612       +8     

Flag	Coverage Δ
unittests	45.60% <50.00%> (+0.04%)	⬆️

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[XiShanYongYe-Chang]

/assign