Minor code refactoring for explain (opensearch-project#2669)

Signed-off-by: Neetika Singhal <neetiks@amazon.com>

Author: neetikasinghal

src/main/java/org/opensearch/knn/index/query/KNNWeight.java

@@ -128,6 +128,8 @@ public Explanation explain(LeafReaderContext context, int doc, float score) {
         knnQuery.setExplain(true);
         try {
             final KNNScorer knnScorer = getOrCreateKnnScorer(context);
+            // calculate score only when its 0 as for disk-based search,
+            // score will be passed from the caller and there is no need to re-compute the score
             if (score == 0) {
                 score = getKnnScore(knnScorer, doc);
             }

src/test/java/org/opensearch/knn/index/ExplainIT.java

@@ -43,7 +43,7 @@
 public class ExplainIT extends KNNRestTestCase {
 
     @SneakyThrows
-    public void testAnnSearch() {
+    public void testExplain_whenDefault_thenANNSearch() {
         int dimension = 128;
         int numDocs = 100;
         createDefaultKnnIndex(dimension);
@@ -63,7 +63,7 @@ public void testAnnSearch() {
     }
 
     @SneakyThrows
-    public void testANNWithExactSearch() {
+    public void testExplain_whenFilterIdLessThanK_thenANNWithExactSearch() {
         createDefaultKnnIndex(2);
         indexTestData(INDEX_NAME, FIELD_NAME, 2, 2);
 
@@ -90,7 +90,7 @@ public void testANNWithExactSearch() {
     }
 
     @SneakyThrows
-    public void testRadialWithANNSearch() {
+    public void testExplain_whenDefaultWithDist_thenRadialWithANNSearch() {
         int dimension = 128;
         int numDocs = 100;
         createDefaultKnnIndex(dimension);
@@ -125,7 +125,7 @@ public void testRadialWithANNSearch() {
     }
 
     @SneakyThrows
-    public void testRadialWithExactSearch() {
+    public void testExplain_whenFilerQueryWithDist_thenRadialWithExactSearch() {
         setupKNNIndexForFilterQuery();
 
         final float[] queryVector = new float[] { 3.3f, 3.0f, 5.0f };
@@ -159,7 +159,7 @@ public void testRadialWithExactSearch() {
     }
 
     @SneakyThrows
-    public void testDiskBasedSearchWithDefaultRescoring() {
+    public void testExplain_whenDefaultDiskBasedSearch_thenRescoringEnabled() {
         int dimension = 16;
         float[] queryVector = new float[] {
             1.0f,
@@ -217,7 +217,7 @@ public void testDiskBasedSearchWithDefaultRescoring() {
     }
 
     @SneakyThrows
-    public void testDiskBasedSearchWithRescoringDisabled() {
+    public void testExplain_whenDiskBasedSearchRescoringDisabled_thenSucceed() {
         int dimension = 16;
         float[] queryVector = new float[] {
             1.0f,