kv, tests: use failpoint to mock the failed KV saving

[JmPotato]

Signed-off-by: JmPotato ghzpotato@gmail.com

What problem does this PR solve?

Part of #3453. Trying to sort out the usage of core/Storage and kv/Base.

What is changed and how it works?

There is no need to introduce a testErrorKV here. I think injecting a failpoint is a more suitable way.

Check List

Tests

• Unit test
• Integration test

Release note

None.

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[disksing]

/merge

[ti-chi-bot]

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/run-all-tests

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[ti-chi-bot]

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

Commit hash: 531477f

[codecov]

Codecov Report

Merging #4275 (531477f) into master (f2b196f) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4275      +/-   ##
==========================================
- Coverage   75.05%   74.82%   -0.24%     
==========================================
  Files         264      264              
  Lines       27253    27255       +2     
==========================================
- Hits        20456    20394      -62     
- Misses       4996     5041      +45     
- Partials     1801     1820      +19     

Flag	Coverage Δ
unittests	74.82% <100.00%> (-0.24%)	⬇️

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

Impacted Files	Coverage Δ
server/kv/etcd_kv.go	70.42% <100.00%> (-9.29%)	⬇️
pkg/errs/errs.go	75.00% <0.00%> (-25.00%)	⬇️
server/tso/tso.go	60.22% <0.00%> (-7.96%)	⬇️
pkg/dashboard/adapter/manager.go	79.78% <0.00%> (-3.20%)	⬇️
server/tso/allocator_manager.go	63.50% <0.00%> (-2.84%)	⬇️
server/server.go	70.20% <0.00%> (-2.40%)	⬇️
server/election/lease.go	86.15% <0.00%> (-1.54%)	⬇️
server/schedule/operator_controller.go	82.68% <0.00%> (-1.23%)	⬇️
server/encryptionkm/key_manager.go	72.80% <0.00%> (-0.80%)	⬇️
... and 6 more

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