There is no purpose to mining Testnet. Please don't.
If you need to "Test a mining rig", then use Testnet in a Box (http://sourceforge.net/projects/bitcoin/files/Bitcoin/testnet-in-a-box/
Testnet-in-a-Box has multiple advantages over the real Testnet:
1) The difficulty is 1, so you find blocks almost immediately.
2) It makes sure you don't impact either the Bitcoin network or the actual Testnet
3) It has all of the same advantages as the real Testnet.
I use Testnet-in-a-Box to actually test my mining pools, so I can verify payouts are working, shares are being handed out, etc.
I don't have to compete against anyone else, so it's perfect for doing my own development.