I'd like to make a copy of my 2017 return for testing various scenario's (my return this year will be very similar) but the copy function is missing.

Unfortunately, the copy function has been removed.

You could consider transferring your 2017 info into a new 2018 return to play around with. That way, you'll get the benefit of the 2018 tax rates. There's no cost since with online editions, you only pay when you're ready to file.

Then, if you'd like to start fresh when you're ready to prepare your return for real, just start a new return in your account and do a transfer from your 2017 file again.