[release-3.6] e2e: add upgrade test for clusters set up by promoted members

[fuweid]

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

Backport: #19634)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.86%. Comparing base (de774e7) to head (a1c9687).
Report is 10 commits behind head on release-3.6.

Additional details and impacted files

see 29 files with indirect coverage changes

@@               Coverage Diff               @@
##           release-3.6   #19662      +/-   ##
===============================================
- Coverage        68.98%   68.86%   -0.13%     
===============================================
  Files              420      420              
  Lines            35827    35784      -43     
===============================================
- Hits             24717    24644      -73     
- Misses            9689     9711      +22     
- Partials          1421     1429       +8     

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[ivanvc]

/retitle [release-3.6] e2e: add upgrade test for clusters set up by promoted members

🙈