tests/etcdutil: reduce etcdutil test time

[lhy1024]

What problem does this PR solve?

Issue Number: ref #7930

What is changed and how does it work?

Check List

Tests

• Unit test

local

Before 
go test -timeout 120s -run ^TestLoopWatcherTestSuite$ -testify.m ^(TestWatcherLoadLargeKey)$ github.com/tikv/pd/pkg/utils/etcdutil
ok      github.com/tikv/pd/pkg/utils/etcdutil   51.058s
After
go test -timeout 120s -run ^TestLoopWatcherTestSuite$ -testify.m ^(TestWatcherLoadLargeKey)$ github.com/tikv/pd/pkg/utils/etcdutil
ok      github.com/tikv/pd/pkg/utils/etcdutil   28.717s

ci

Before 
https://github.com/tikv/pd/actions/runs/8339040119/job/22820335714
ok  	http://github.com/tikv/pd/pkg/utils/etcdutil	208.548s 
After
https://github.com/tikv/pd/actions/runs/8339647034/job/22822028949?pr=7945
ok  	http://github.com/tikv/pd/pkg/utils/etcdutil	125.064s

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• HuSharp
• JmPotato

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[HuSharp]

why need this function?

[codecov]

Codecov Report

Merging #7945 (5a03669) into master (3fe806a) will increase coverage by 0.08%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7945      +/-   ##
==========================================
+ Coverage   73.50%   73.58%   +0.08%     
==========================================
  Files         436      436              
  Lines       48362    48363       +1     
==========================================
+ Hits        35548    35589      +41     
+ Misses       9755     9721      -34     
+ Partials     3059     3053       -6     

Flag	Coverage Δ
unittests	73.58% <100.00%> (+0.08%)	⬆️

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[JmPotato]

/merge

[ti-chi-bot]

@JmPotato: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5a03669