[MINOR] Move a test method to Transformations (#1934)

- Move TestHoodieKeyLocationFetchHandle#getRecordsPerPartition to Transformations
- Improve some var namings

hudi-common/src/test/java/org/apache/hudi/common/testutils/Transformations.java

@@ -25,9 +25,12 @@ import org.apache.hudi.common.model.HoodieRecord;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
 
+import static java.util.stream.Collectors.groupingBy;
+
 /**
  * Common transformations in test cases.
  */
@@ -51,6 +54,10 @@ public final class Transformations {
     return records.stream().map(HoodieRecord::getKey).collect(Collectors.toList());
   }
 
+  public static Map<String, List<HoodieRecord>> recordsToPartitionRecordsMap(List<HoodieRecord> records) {
+    return records.stream().collect(groupingBy(HoodieRecord::getPartitionPath));
+  }
+
   /**
    * Pseudorandom: select even indices first, then select odd ones.
    */