fix ut in TestMultiClusterCache watch

[ikaven1024]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes:
Refer to #3730 (comment)

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #3862 (fda27d9) into master (2526de8) will increase coverage by 0.01%.
Report is 6 commits behind head on master.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #3862      +/-   ##
==========================================
+ Coverage   55.17%   55.19%   +0.01%     
==========================================
  Files         227      227              
  Lines       21653    21671      +18     
==========================================
+ Hits        11948    11962      +14     
- Misses       9069     9072       +3     
- Partials      636      637       +1     

Flag	Coverage Δ
unittests	55.19% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 5 files with indirect coverage changes

[ikaven1024]

The behavior of Cacher handling watch requests with empty RV changes. It will fetch objects from underlying storage, instead of local cache.

This change is introduced fron kubernetes/kubernetes#115096

@XiShanYongYe-Chang @RainbowMango

[RainbowMango]

/lgtm
/approve

Great thanks! @ikaven1024

[karmada-bot]

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