Support disabling NoteDb reads for tests

In I0a9eb0ad we fixed a NoteDb-specific issue where loading the
default search results page caused ChangeNotes to get loaded, despite
not causing changes to load from ReviewDb.

Support this configuration in tests by adding a method to
NotesMigration that is called in AbstractChangeNotes#load(). In
production this method always returns false so the cost is negligible.

Change-Id: I22622cb3c834eeb1391d5b80247beafe36fffedf

gerrit-server/src/test/java/com/google/gerrit/testutil/TestNotesMigration.java

@@ -22,6 +22,7 @@ import com.google.inject.Singleton;
 public class TestNotesMigration extends NotesMigration {
   private volatile boolean readChanges;
   private volatile boolean writeChanges;
+  private volatile boolean failOnLoad;
 
   @Override
   public boolean readChanges() {
@@ -43,6 +44,11 @@ public class TestNotesMigration extends NotesMigration {
     return false;
   }
 
+  @Override
+  public boolean failOnLoad() {
+    return failOnLoad;
+  }
+
   public TestNotesMigration setReadChanges(boolean readChanges) {
     this.readChanges = readChanges;
     return this;
@@ -53,6 +59,11 @@ public class TestNotesMigration extends NotesMigration {
     return this;
   }
 
+  public TestNotesMigration setFailOnLoad(boolean failOnLoad) {
+    this.failOnLoad = failOnLoad;
+    return this;
+  }
+
   public TestNotesMigration setAllEnabled(boolean enabled) {
     return setReadChanges(enabled).setWriteChanges(enabled);
   }