Turn on memcache in flow tests.

No tests break because of this, but some tests that were previously
intentionally clearing memcache (for example, the test in
DomainInfoFlow that counts datastore calls) are now doing something
that is not a no-op anymore.

Also fix an incorrect comment about memcache in DatastoreHelper.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154087182

javatests/google/registry/flows/FlowTestCase.java

@@ -83,6 +83,7 @@ public abstract class FlowTestCase<F extends Flow> extends ShardableTestCase {
   @Rule
   public final AppEngineRule appEngine = AppEngineRule.builder()
       .withDatastore()
+      .withMemcache()
       .withTaskQueue()
       .build();