You can run the testnet along side your primary wallet, they will not conflict with each other. There is no need to specify a data directory, it will create a subfolder in the default data directory and the chain, wallet, etc. will be there. The only shared file will be the darknet.conf.
if you have an (idiot proved) mac guide for this
i consider
You just start the wallet with the --testnet flag. So, for the qt, ./darknet-qt --testnet
Edit: There may be a slightly different syntax for starting programs on Mac than Linux, let me yell to s3v3n, he is the Mac guy.