The test file compiler fails if one uses --disable-vp8-decoder
--enable-vp9-decoder. It effectively turns on CONFIG_VP8 and
CONFIG_DECODERS, but turns off CONFIG_VP8_DECODER, which causes
compiler error at test_vector_test.cc.

This commit fixes this issue by adding vp8/9 decoder flags to
the decoder behavior test, respectively.

Change-Id: I097ff8fd5e12715a94a565a82e54503885eb7187