[Refactor] Move exact search specific test from KNNWeightTests to ExactSearcherTests

[VijayanB]

Currently, we use KNNWeightTests to test exact search methods for unit test. Since Exact Search had one function, it was no needed to have it's own Test class. Now we are adding more features to exact search, hence, it is better mock ExactSearcher in KNNWeightTests and add unit test inside ExactSearcherTest. This will improve readability and visibility of unit test whenever new feature is added.

Example

1. https://github.com/opensearch-project/k-NN/blob/main/src/test/java/org/opensearch/knn/index/query/KNNWeightTests.java#L679
2. https://github.com/opensearch-project/k-NN/blob/main/src/test/java/org/opensearch/knn/index/query/KNNWeightTests.java#L684