[Backport 2.x] Adding test for KNNWeight

[opensearch-trigger-bot]

Backport 5b06cfe from #759

[codecov-commenter]

Codecov Report

Merging #763 (dafdf89) into 2.x (7ba9088) will increase coverage by 0.29%.
The diff coverage is n/a.

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@@             Coverage Diff              @@
##                2.x     #763      +/-   ##
============================================
+ Coverage     84.88%   85.18%   +0.29%     
- Complexity     1079     1083       +4     
============================================
  Files           151      151              
  Lines          4380     4380              
  Branches        393      393              
============================================
+ Hits           3718     3731      +13     
+ Misses          482      472      -10     
+ Partials        180      177       -3     

Impacted Files	Coverage Δ
...main/java/org/opensearch/knn/index/util/Faiss.java	75.51% <0.00%> (+1.02%)	⬆️
...java/org/opensearch/knn/index/query/KNNWeight.java	89.47% <0.00%> (+10.52%)	⬆️
.../main/java/org/opensearch/knn/index/SpaceType.java	81.25% <0.00%> (+12.50%)	⬆️

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