tests: Migrate completion tests to common framework

[kkkkun]

Part of #13637 (comment)

[codecov-commenter]

Codecov Report

Merging #13825 (0a33d5c) into main (1b208aa) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #13825      +/-   ##
==========================================
- Coverage   72.63%   72.54%   -0.10%     
==========================================
  Files         467      467              
  Lines       38280    38280              
==========================================
- Hits        27806    27769      -37     
- Misses       8686     8710      +24     
- Partials     1788     1801      +13     

Flag	Coverage Δ
all	72.54% <ø> (-0.10%)	⬇️

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

Impacted Files	Coverage Δ
client/v3/txn.go	73.33% <0.00%> (-26.67%)	⬇️
client/v3/leasing/cache.go	87.77% <0.00%> (-3.89%)	⬇️
server/etcdserver/api/v3rpc/member.go	93.54% <0.00%> (-3.23%)	⬇️
client/v3/leasing/txn.go	88.09% <0.00%> (-3.18%)	⬇️
client/v3/concurrency/election.go	79.68% <0.00%> (-2.35%)	⬇️
server/etcdserver/api/rafthttp/msgappv2_codec.go	69.56% <0.00%> (-1.74%)	⬇️
server/etcdserver/api/v3rpc/maintenance.go	76.71% <0.00%> (-1.37%)	⬇️
client/v3/op.go	74.32% <0.00%> (-1.15%)	⬇️
server/storage/mvcc/watchable_store.go	88.76% <0.00%> (-1.09%)	⬇️
client/v3/leasing/kv.go	90.36% <0.00%> (-1.00%)	⬇️
... and 10 more

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[serathius]

This PR refactors the code, however doesn't do the test migration. Goal of the migration is to use single framework for unit and integration tests.

Please skip the tests that operate on binaries for now.

[kkkkun]

This PR refactors the code, however doesn't do the test migration. Goal of the migration is to use single framework for unit and integration tests.

Please skip the tests that operat on binaries for now.

That's ok.

[serathius]

I'm ok with the refactor, however let's leave the code in tests/e2e directory.