To create a mock return in Online TurboTax you would have to set up a second online account to use as a "test account". It won't charge you unless you decide to print.
Do NOT experiment in your current account with your already-filed return. TurboTax saves your return after each screen, so you would overwrite your current return.
People using desktop software (CD/download) can prepare as many test returns as they want, or can use the "What-if" worksheet to compare situations. That feature is not available in Online TurboTax.