KEP-4578: make the featuragate test more extensible.

[liangyuanpeng]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

part of #18023

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Codecov Report

Attention: Patch coverage is 6.89655% with 27 lines in your changes missing coverage. Please review.

Project coverage is 68.82%. Comparing base (93a245a) to head (c518fd3).
Report is 22 commits behind head on main.

❗ Current head c518fd3 differs from pull request most recent head a2be729

Please upload reports for the commit a2be729 to get more accurate results.

Files with missing lines	Patch %	Lines
client/v3/kubernetes/client.go	0.00%	27 Missing ⚠️

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Additional details and impacted files

Files with missing lines	Coverage Δ
server/storage/mvcc/kvstore.go	89.76% <100.00%> (ø)
client/v3/kubernetes/client.go	0.00% <0.00%> (ø)

... and 21 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18508      +/-   ##
==========================================
- Coverage   68.83%   68.82%   -0.01%     
==========================================
  Files         420      420              
  Lines       35489    35470      -19     
==========================================
- Hits        24429    24413      -16     
+ Misses       9639     9628      -11     
- Partials     1421     1429       +8     

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[serathius]

Any reason for appends here?

[liangyuanpeng]

Add a comment for it, how aboud now.

// add a new experimental flag here when migrating a new experimental flag to feature.

[serathius]

I meant that why some cases are defined as at list creation and why some are appends? Append is more concise, could we use it for declaring the whole testCases slice?