Make DatastoreHelper set creation time to START_OF_TIME for contacts

It was already doing this for domains and hosts, and that it wasn't doing it for
contacts was confusing and caused me to experience a difficult-to-track-down
error in writing new code.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134827468

javatests/google/registry/testing/DatastoreHelper.java

@@ -218,6 +218,7 @@ public class DatastoreHelper {
         .setContactId(contactId)
         .setCurrentSponsorClientId("TheRegistrar")
         .setAuthInfo(ContactAuthInfo.create(PasswordAuth.create("2fooBAR")))
+        .setCreationTimeForTest(START_OF_TIME)
         .build();
   }