Hm, adding --enable-testnet to ./configure doesn't seem to be working. I did it for libbitcoin, obelisk, and sx. It is still giving me the following, which is the hash of mainnet genesis block. Yes I am sure I am using the correct sx binary.
~/sx/worker$ ../bin/sx initchain blockchain/
INFO: Imported genesis block 000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f
...