Fix the superframe unit test for ext-refs

Change-Id: I64b236d859a50a265b9b9ee2315bf2549e63881d

test/superframe_test.cc

@@ -96,7 +96,13 @@ TEST_P(SuperframeTest, TestSuperframeIndexIsOptional) {
   ::libaom_test::I420VideoSource video("hantro_collage_w352h288.yuv", 352, 288,
                                        30, 1, 0, 40);
   ASSERT_NO_FATAL_FAILURE(RunLoop(&video));
+#if CONFIG_EXT_REFS
+  // NOTE: The use of BWDREF_FRAME will enable the coding of more non-show
+  //       frames besides ALTREF_FRAME.
+  EXPECT_GE(sf_count_, 1);
+#else
   EXPECT_EQ(sf_count_, 1);
+#endif  // CONFIG_EXT_REFS
 }
 
 #if CONFIG_ANS