Like I would do
wget -O - http://blockchain.info/q/getblockcount 2>/dev/null for the main net.
Blockchain.info does not track the testnet, but blockexplorer.com does. In fact the API you're using was originally from blockexplorer.com, and blockchain.info simply adopted it.
wget -O - http://blockexplorer.com/testnet/q/getblockcount 2> /dev/null