You may find one of these auxiliary fuel bottles handy.
https://www.ebay.co.uk/itm/MOTORCYCL...EAAOSwOddYxs-n
Obviously you could only do a static test using one of these (and even then, not at sustained high revs) but it would ensure enough gravity feed pressure as well as eliminating everything else related to the std tank (pump, filter, tap, breather, etc).