As far as I know you can still do your own airtests - sole owner etc.
That said I think that it is of benefit to have an experienced airtester along as the observer simply because we see lots of different types andmight be able to provide valuable info.
I am always concious of the fact that many owners do not want some unknown flying their pride and joy so I will always offer to observe instead.
In that case it is also a perfect opportunity to sign off your Dual Flight with Instructor too (at no extra charge)!