Centralize creation of MapreduceRunner in unit tests

This also changes the default number of mapper shards in tests to 2, which is
the number of EppResourceIndex buckets in unit tests. Running more shards than
there are buckets causes unnecessary test load.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135520601

javatests/google/registry/flows/async/DnsRefreshForHostRenameActionTest.java

@@ -26,11 +26,9 @@ import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
 import static org.mockito.Mockito.verifyZeroInteractions;
 
-import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableSet;
 import com.googlecode.objectify.Key;
 import google.registry.dns.DnsQueue;
-import google.registry.mapreduce.MapreduceRunner;
 import google.registry.model.host.HostResource;
 import google.registry.request.HttpException.BadRequestException;
 import google.registry.testing.ExceptionRule;
@@ -64,7 +62,7 @@ public class DnsRefreshForHostRenameActionTest
   private void runMapreduce(String hostKeyString) throws Exception {
     action = new DnsRefreshForHostRenameAction();
     action.hostKeyString = hostKeyString;
-    action.mrRunner = new MapreduceRunner(Optional.<Integer>of(5), Optional.<Integer>absent());
+    action.mrRunner = makeDefaultRunner();
     action.response = new FakeResponse();
     action.run();
     executeTasksUntilEmpty("mapreduce");