So then I assume a M class holder (2wheel bike) is allowed to ride the slingshot with no issues..?
The slingshot requires a G class to operate actually but are registered as a motorcycle.
Spyder (or any other trikes) can be operated with any M class licence.
Your question was specific to taking the road test in a three wheeled vehicle however, in the resulting restricted license that would come with it.
Arguably, come to think of it, the MTO could potentially disqualify you from taking your road test in a Slingshot given as how it is classified as a G class vehicle, actually, but the fact it's registered as a motorcycle on the ownership would give you a point to argue it.
I suspect it would be dependent on how well versed the examiner is on the Slingshot, if he/she knows that it's actually considered a G class vehicle despite being a 3 wheeler with helmets that many would lump in with motorcycles a a result. The fact it's registered as a motorcycle on the ownership would just confuse things even more I would think.
I suspect the same potential confusion would come into play if somebody arrived for a G road test in a slingshot - it's mixed breed licence/registration situation just muddies the waters.
In the end instead of dealing with all the possible confusion, license restrictions, and potentially being refused the test completely, just do it on your motorcycle instead.