Turn on error resilient mode in frame drop unit test

Turn on the error resilient mode in the frame dropping unit test.
The temporal mv reference will be disabled in error resilient mode
by design.

BUG=aomedia:1411

Change-Id: Id777061bbdc376f8d1634c1286fce8094fdcc04e
diff --git a/test/datarate_test.cc b/test/datarate_test.cc
index 9d723c8..4ad3101 100644
--- a/test/datarate_test.cc
+++ b/test/datarate_test.cc
@@ -215,6 +215,7 @@
   cfg_.rc_end_usage = AOM_CBR;
   cfg_.rc_target_bitrate = 200;
   cfg_.g_lag_in_frames = 0;
+  cfg_.g_error_resilient = 1;
   // TODO(marpan): Investigate datarate target failures with a smaller keyframe
   // interval (128).
   cfg_.kf_max_dist = 9999;