improve overridemanager test

[vie-serendipity]

What type of PR is this?

/kind flake
What this PR does / why we need it:
Actually, this doesn't trigger any bugs, but it ought to be of the form `"{{string}}"`. Otherwise there will be no double quotes when converting to a string and cause a marshal error in encoding/json. Just in case developers want to add a unit test on plaintext overrider later, I create this pr.
Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

None

[karmada-bot]

Welcome @vie-serendipity! It looks like this is your first PR to karmada-io/karmada 🎉

[XiShanYongYe-Chang]

Hi @vie-serendipity welcome to karmada!
You can sign off refer to https://github.com/karmada-io/karmada/pull/3943/checks?check_run_id=15900231424

[XiShanYongYe-Chang]

Thanks a lot.
The updated context LGTM

[RainbowMango]

/approve

Thanks @vie-serendipity
You might need to sign the commit as per: #3943 (comment)

/assign @XiShanYongYe-Chang

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: RainbowMango

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• OWNERS [RainbowMango]

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[codecov-commenter]

Codecov Report

Merging #3943 (ec6a83c) into master (25db1e0) will decrease coverage by 0.02%.
Report is 8 commits behind head on master.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #3943      +/-   ##
==========================================
- Coverage   54.71%   54.69%   -0.02%     
==========================================
  Files         229      229              
  Lines       22074    22074              
==========================================
- Hits        12077    12074       -3     
- Misses       9351     9354       +3     
  Partials      646      646              

Flag	Coverage Δ
unittests	54.69% <ø> (-0.02%)	⬇️

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

see 1 file with indirect coverage changes

[RainbowMango]

/lgtm