Fix incorrect assertions in CollapsingTopDocsCollectorSearchAfterTests

These assertions were simply wrong, but took time to catch as they were very
rarely triggered.

Fixes elastic#75736.

server/src/test/java/org/apache/lucene/grouping/CollapsingTopDocsCollectorSearchAfterTests.java

@@ -133,10 +133,11 @@ private <T extends Comparable<T>> void assertSearchCollapse(CollapsingDocValuesP
             currentValue = fieldDoc.fields[0];
         }
 
-        if (docsWithMissingField == false) {
-            assertEquals(expectedNumGroups, collapseTopFieldDocs.scoreDocs.length - 1);
-            assertEquals(topDocs.scoreDocs.length - 1, topDocsIndex);
+        for (; topDocsIndex < topDocs.scoreDocs.length; topDocsIndex++) {
+            FieldDoc fieldDoc = (FieldDoc) topDocs.scoreDocs[topDocsIndex];
+            assertEquals(fieldDoc.fields[0], currentValue);
         }
+
         w.close();
         reader.close();
         dir.close();