simple_decoder.sh: Support encoding in decode test scripts.

Adding AV1 input files to the test set is not feasible because the
bitstream is in constant flux. Add test input encoding and hook
it up in simple_decoder.sh to start.

cherry-picked #b591df89 from aom/master

Change-Id: Ie4c06a7c458cdc2ab003d27fb92418c77c87fc88