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/batch/DeleteContactsAndHostsActionTest.java

@@ -142,6 +142,7 @@ public class DeleteContactsAndHostsActionTest
     createTld("tld");
     clock.advanceOneMilli();
   }
+
   @Test
   public void testSuccess_contact_referencedByActiveDomain_doesNotGetDeleted() throws Exception {
     ContactResource contact = persistContactPendingDelete("blah8221");