[fix][test] Update partitioned topic subscription assertions in IncrementPartitionsTest

[kontotto]

Motivation

The previous assertion only verified subscriptions for partition 15. This change expands the scope to cover all partitions, ensuring comprehensive validation of subscription behavior.

Modifications

Fix assertion to iterate through all partitions and verify subscriptions, ensuring comprehensive coverage instead of checking only partition 15.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: kontotto#5

[hrsakai]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.26%. Comparing base (bbc6224) to head (73ef211).
Report is 955 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #24056      +/-   ##
============================================
+ Coverage     73.57%   74.26%   +0.68%     
+ Complexity    32624     2379   -30245     
============================================
  Files          1877     1861      -16     
  Lines        139502   144209    +4707     
  Branches      15299    16433    +1134     
============================================
+ Hits         102638   107090    +4452     
+ Misses        28908    28693     -215     
- Partials       7956     8426     +470     

Flag	Coverage Δ
inttests	26.67% <ø> (+2.08%)	⬆️
systests	23.11% <ø> (-1.22%)	⬇️
unittests	73.77% <ø> (+0.92%)	⬆️

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see 1056 files with indirect coverage changes

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