id say book your test, there's usually a bit of a wait anyway, then discuss it with your instructor, its something to work towards, or discuss it first and then book it, but get it booked
you don't want to needlessly pay for lessons if you're up to test standard within the coming weeks and need to wait 6 or 8 weeks for a test, a real possibility around this time. its coming up to summer now, test centers are going to be getting busier as the weeks go on as people want to pass before the summer, before they go to uni etc any cancellations will be snapped up in a heart beat, its like a feeding frenzy at certain times of the year.
You can always push back a test if you're not ready, but personally id book a test for in say 4 or 6 weeks, have that as an aim, if you need more lessons, either try to fit them in or move the test back, there's no harm in it, and if its booked you'll either work and get to the standard or put it back, but at least there wont be wasted weeks between being ready and taking the test.
There's nothing more frustrating than needlessly spending money, lessons are expensive! when you knew that you should have booked a test earlier and worked towards that goal.
whats the worst that can happen? you get within a week of the test, don't feel ready or think you need some extra time and you push it back, its no big deal.