I'm glad to hear that you've had a chance to play around in the Test File for QuickBooks Online. I just took a peek at it myself and I see exactly what you're saying about the options in the drop-down of the sales tax column only showing those three taxes you mentioned. If you look at the Amounts are drop-down on a form though (just above the far right column), you'll see out of scope there. However, the option should also be appearing for the individual line items, so there may be a glitch with the test drive at this time. I'll report it to my team.
Since the test file isn't working quite as it should, I can appreciate wanting to try in a live account before you get started I recommend using a QuickBooks Online 30-day trial to enter data and try things out to see if it's for you. Once you're ready to start, you have two options: you can start a brand new subscription separate from the trial or you can delete all the data in the trial and start fresh once you subscribe.
Things to keep in mind if you're going to use the second option is that any payroll information you enter to test cannot be deleted through this function since payroll is an add-on feature. I'll leave you with this article to learn more about deleting the information: Delete your data and start over in QuickBooks Online
Give me a shout if you need anything else!