Sorry, should be easy to answer 
So, turbo is in ERG and it’s coming to the last ramp, legs are screaming, holding a cadence of about 90 but struggling to hit the power at cadence. No gear changes up to now…so
Do you drop a few gears and lower cadence and grind it or
Increase cadence, probably too difficult or
Give in and stop test?
Gear changes won’t do anything except ruin your rhythm and cause your trainer to make up the difference. Because you’re in ERG mode, when you can no longer complete the intervals at target power, regardless of cadence, that’s when you should quit. Ideally, you’d have your cadence high coming into the last few minutes of the test, and I find that my legs just stop pedaling all by themselves (lower cadence and stop hitting the power target) at the end of that final minute.
Thanks Larry, yeah that’s pretty much how I’m approaching it.
Let us know how your next test goes! 
Like any other cycling test, you need to used to the test.
I’ve done the ramp test 3 or 4 times now. My last one felt the most ‘successful’.
I didn’t mess with anything. Didn’t bother with gear changes. I just kept my butt on the seat and kept pedaling until I wasn’t pedaling anymore. My cadence had gone from the 90’s down to maybe the 70s or 60s and then I was just done.
I’d recommend that you do the same. Just pedal until there’s nothing left in your legs. Don’t worry yourself with anything like shifting. I really think that your oxygen demand at the end of this test is so severe that if you waste your energy thinking about shifting gears your going to quit earlier than where you maybe could have.
Good luck!