Refactor out creation of server TRIDs so they can be tested

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152837185

javatests/google/registry/model/poll/PollMessageExternalKeyConverterTest.java

@@ -70,7 +70,7 @@ public class PollMessageExternalKeyConverterTest {
       .setXmlBytes("<xml></xml>".getBytes(UTF_8))
       .setModificationTime(clock.nowUtc())
       .setClientId("foo")
-      .setTrid(Trid.create("ABC-123"))
+      .setTrid(Trid.create("ABC-123", "server-trid"))
       .setBySuperuser(false)
       .setReason("reason")
       .setRequestedByRegistrar(false)