Add forgotten test_decoder.c.
Add forgotten test_decoder.c.

tbetbetbe/upb,tbetbetbe/upb,bsn069/upb,tempbottle/upb,tempbottle/upb,tbetbetbe/upb,bsn069/upb,tempbottle/upb,tempbottle/upb,bsn069/upb,tbetbetbe/upb,bsn069/upb,tempbottle/upb,bsn069/upb,tbetbetbe/upb