I think this is one of those things which varies from country to country and even state to state. It would certainly be an interesting question to see answered, although many of the contributors here have younger children.
My eldest was able to do adult entry to uni via the STAT test once she turned 18, but that was for South Australia. For other states you have to be 21 before you can go that route. Otherwise you need a year 12 certificate and your results will dictate which courses you can get onto. If you don't have good enough results for the course you want, then you can start on another course and do the required units to change over after a year.
Admittedly, my girls have had some experience of tests from school and my own Miss 17 is currently doing her SACE via open access, so has had a couple of tests from this. This year she'll have to attend an actual exam at the college, though.
I suppose mock tests would be a way to start to familiarise homeschooled students with the feel of them if they want to head on to formal education.