Increment the clock in expand recurring billings events test

Also adds some clarity on when to use the non-incrementing version.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143037508

javatests/google/registry/testing/DatastoreHelper.java

@@ -643,7 +643,6 @@ public class DatastoreHelper {
             FluentIterable.from(asList(expected)).transform(BILLING_EVENT_ID_STRIPPER));
   }
 
-
   /** Assert that there are no billing events. */
   public static void assertNoBillingEvents() {
     assertThat(getBillingEvents()).isEmpty();