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/main/java/com/google/gerrit/server/notedb/NotesMigration.java

@@ -42,6 +42,15 @@ public abstract class NotesMigration {
 
   public abstract boolean writeAccounts();
 
+  /**
+   * Whether to fail when reading any data from NoteDb.
+   * <p>
+   * Used in conjunction with {@link #readChanges()} for tests.
+   */
+  public boolean failOnLoad() {
+    return false;
+  }
+
   public boolean enabled() {
     return writeChanges() || readChanges()
         || writeAccounts() || readAccounts();