Create test helper class, ChangesForTestUtil

Some methods necessary for the changes in CommentsTest were private to
ChangeNotes, so in order to modularize, I factored those common
methods out into a new testutil helper class, ChangesForTestUtil. This
utility class helps to generate ChangeUpdates, Changes, and
ChangeControls for testing purposes.

Change-Id: Ic2456eab131d394881374db16503144c52118fb0

gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNotes.java

@@ -520,7 +520,7 @@ public class ChangeNotes extends VersionedMetaData {
   NoteMap noteMap;
 
   @VisibleForTesting
-  ChangeNotes(GitRepositoryManager repoManager, Change change) {
+  public ChangeNotes(GitRepositoryManager repoManager, Change change) {
     this.repoManager = repoManager;
     this.change = new Change(change);
   }