Refactor ForwardIndexHandlerTest and avoid using setters for IndexLoadingConfig

[Jackie-Jiang]

Sub-task of #14106

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.09%. Comparing base (59551e4) to head (cc0f7ec).
Report is 1110 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #14124      +/-   ##
============================================
+ Coverage     61.75%   64.09%   +2.33%     
- Complexity      207     1537    +1330     
============================================
  Files          2436     2600     +164     
  Lines        133233   143505   +10272     
  Branches      20636    21982    +1346     
============================================
+ Hits          82274    91975    +9701     
+ Misses        44911    44752     -159     
- Partials       6048     6778     +730     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (+99.99%)	⬆️
integration	100.00% <ø> (+99.99%)	⬆️
integration1	100.00% <ø> (+99.99%)	⬆️
integration2	0.00% <ø> (ø)
java-11	64.03% <ø> (+2.32%)	⬆️
java-21	63.98% <ø> (+2.35%)	⬆️
skip-bytebuffers-false	64.06% <ø> (+2.31%)	⬆️
skip-bytebuffers-true	63.95% <ø> (+36.23%)	⬆️
temurin	64.09% <ø> (+2.33%)	⬆️
unittests	64.08% <ø> (+2.34%)	⬆️
unittests1	55.79% <ø> (+8.90%)	⬆️
unittests2	34.53% <ø> (+6.80%)	⬆️

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