pkg/webhook: refactor Multi-Cluster Service test

[mohamedawnallah]

Description

In this commit, we refactor the multi-cluster service object names and check on number of patches after the mutation happens.

Motivation and Context

Motivated by @XiShanYongYe-Chang's comment #5494 (comment).

What type of PR is this?

/kind cleanup

Which issue(s) this PR fixes:
Follow-up on #5517.

Does this PR introduce a user-facing change?:

NONE

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.04%. Comparing base (bf83dfe) to head (47290cb).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5539   +/-   ##
=======================================
  Coverage   34.03%   34.04%           
=======================================
  Files         643      643           
  Lines       44509    44509           
=======================================
+ Hits        15150    15151    +1     
- Misses      28205    28206    +1     
+ Partials     1154     1152    -2     

Flag	Coverage Δ
unittests	34.04% <ø> (+<0.01%)	⬆️

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