When you're talking about "testing the blockchain". Do you mean you've created your own blockchain from scratch, or do you mean testing an application that used one of the existing crypto currencies  to accept payments, store smart contracts in a blockchain or store data in a blockchain?

If it's the first case (built your own blockchain): If you developed your own blockchain from scratch, this question should be very easy for you...

If it's the second case (using an existing blockchain): Most establised crypto currencies have a main net, a testnet and regtest mode. The testnet and regtest are usually used for testing new software applications.