tests: Refactor etcd model

[serathius]

cc @ahrtr

[serathius]

@ahrtr Please let me know what should could be improved in documenting model.

[codecov-commenter]

Codecov Report

Merging #15045 (922bed1) into main (16e1fff) will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #15045      +/-   ##
==========================================
- Coverage   74.75%   74.60%   -0.16%     
==========================================
  Files         415      415              
  Lines       34273    34273              
==========================================
- Hits        25622    25568      -54     
- Misses       7015     7055      +40     
- Partials     1636     1650      +14     

Flag	Coverage Δ
all	74.60% <ø> (-0.16%)	⬇️

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

Impacted Files	Coverage Δ
client/pkg/v3/tlsutil/tlsutil.go	83.33% <0.00%> (-8.34%)	⬇️
client/v3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
client/v3/concurrency/mutex.go	61.03% <0.00%> (-5.20%)	⬇️
server/storage/mvcc/kvstore_compaction.go	95.65% <0.00%> (-4.35%)	⬇️
client/v3/leasing/txn.go	88.09% <0.00%> (-3.18%)	⬇️
server/proxy/grpcproxy/watch.go	93.64% <0.00%> (-2.90%)	⬇️
server/etcdserver/api/v3election/election.go	66.66% <0.00%> (-2.78%)	⬇️
client/v3/experimental/recipes/key.go	75.34% <0.00%> (-2.74%)	⬇️
server/etcdserver/api/rafthttp/peer.go	85.06% <0.00%> (-1.95%)	⬇️
pkg/traceutil/trace.go	96.15% <0.00%> (-1.93%)	⬇️
... and 12 more

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