pkg/search/backendstore: unit test opensearch

[mohamedawnallah]

Description

In this commit, we unit test opensearch backend store for karmada search aggregated apiserver as cache registry on creating new instance and getting/creating an index.

What type of PR is this?

Which issue(s) this PR fixes:
Part of #5491.
Depends on #5811.

Does this PR introduce a user-facing change?:

NONE

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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• pkg/search/OWNERS
• pkg/util/OWNERS

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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 75.75758% with 8 lines in your changes missing coverage. Please review.

Project coverage is 45.73%. Comparing base (3acc14c) to head (9b82211).

Files with missing lines	Patch %	Lines
pkg/search/backendstore/opensearch.go	79.16%	5 Missing ⚠️
pkg/search/controller.go	33.33%	2 Missing ⚠️
pkg/util/testing/mock_opensearch.go	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5824      +/-   ##
==========================================
+ Coverage   45.56%   45.73%   +0.17%     
==========================================
  Files         658      659       +1     
  Lines       53921    53929       +8     
==========================================
+ Hits        24567    24664      +97     
+ Misses      27750    27663      -87     
+ Partials     1604     1602       -2     

Flag	Coverage Δ
unittests	45.73% <75.75%> (+0.17%)	⬆️

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