Rear-mounted pickups have been A Thing for decades and several 'parts' companies will make you a body for it, and there are lots on eBay. Grab one HH body and one SSS body, buy identical necks and use Liberator pots (probably best to have a set of active pots on hand, too) and you're good to go. No need to change strings, height or shift the bridge at all when changing pickups.
That said, personally I'd get busy and totally remove the middle block of a body and use pickguard blanks to cut humbucker, Strat, Tele (both neck and bridge) and minibucker mounts which could then be swapped around in any configuration, from behind. Not sure how you could work in soapbar P-90s, but you'd have most combinations to hand.
The bigger problem I see with making one 'test' guitar is just that there can be such a difference between how a pickup feels in set neck mahogany vs bolt-on alder vs neck-through maple that there isn't really one construction and material combination that is truly neutral and universal. Maybe a 2-ply body made of a limba/korina back layer and an ash top layer, then an all-bubinga neck & fretboard that is set in but with a very short tenon, like in a Les Paul DC Special... but then how'd you test pickups in a way that is relevant for hollow bodies?
FWIW I have two test guitars: a Les Paul Vixen, which is a slightly thinner and lighter LP, for testing humbuckers; and a Squier Mustang with a couple of pickguards cut for Strat-style and Tele-style pickups (one SSS, one TB-TN, one TB-TB) that I can just about swap with only slacking off the strings and not having to actually touch the bridge. The Squier's very 'dead' body (some kind of multi-ply basswood) and the Vixen's intonation-unfriendly wraparound bridge do not make them ideal, but they get the basics done better than any one single guitar can.