Re-enable unit test on neon implementation of 16x16 idct

The previous change only tunes forward transform. It doesn't affect
the neon implementation of the inverse transform. Hence turn the
unit test on.

Change-Id: I4f0f43783b98814d1eee53182209f9669d538140

test/dct16x16_test.cc

@@ -512,9 +512,7 @@ INSTANTIATE_TEST_CASE_P(
         make_tuple(&vp9_fht16x16_c, &vp9_iht16x16_256_add_c, 2),
         make_tuple(&vp9_fht16x16_c, &vp9_iht16x16_256_add_c, 3)));
 
-// FIXME (jingning, fgalligan): need to simplify the corresponding steps
-// in neov version accordingly, and re-enable the unit test
-#if HAVE_NEON_ASM && 0
+#if HAVE_NEON_ASM
 INSTANTIATE_TEST_CASE_P(
     NEON, Trans16x16DCT,
     ::testing::Values(