tests: Add tests for snapshot compatibility and recovery between versions

[serathius]

When working on removing V2 store, this tests were critical in finding bugs. They are required that removing v2store will not break backward compatibility.
cc @ptabor @spzala @ahrtr

[codecov-commenter]

Codecov Report

Merging #13756 (6f1e1da) into main (ae57fe5) will decrease coverage by 0.12%.
The diff coverage is 92.00%.

❗ Current head 6f1e1da differs from pull request most recent head 3ffa253. Consider uploading reports for the commit 3ffa253 to get more accurate results

@@            Coverage Diff             @@
##             main   #13756      +/-   ##
==========================================
- Coverage   72.45%   72.33%   -0.13%     
==========================================
  Files         469      469              
  Lines       38276    38283       +7     
==========================================
- Hits        27733    27692      -41     
- Misses       8756     8806      +50     
+ Partials     1787     1785       -2     

Flag	Coverage Δ
all	72.33% <92.00%> (-0.13%)	⬇️

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

Impacted Files	Coverage Δ
etcdutl/etcdutl/backup_command.go	9.94% <0.00%> (ø)
etcdutl/etcdutl/migrate_command.go	75.00% <88.88%> (ø)
etcdutl/etcdutl/defrag_command.go	78.78% <100.00%> (ø)
etcdutl/snapshot/v3_snapshot.go	66.10% <100.00%> (ø)
server/lease/lessor.go	88.80% <100.00%> (ø)
server/storage/backend.go	62.74% <100.00%> (ø)
server/storage/backend/backend.go	83.71% <100.00%> (-0.06%)	⬇️
server/storage/backend/testing/betesting.go	87.50% <100.00%> (ø)
server/verify/verify.go	48.33% <100.00%> (ø)
server/proxy/grpcproxy/auth.go	88.67% <0.00%> (-11.33%)	⬇️
... and 35 more

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[serathius]

cc @ptabor