If you are looking for more opinions on the subject of flood tests - perhaps consult the manufacture of the waterproofing products themselves.
A company like Laticrete based out of Maine is a classic example. In their newsletter they not only recommend flood tests they go on to explain how they are done. Take a quick peak at this news letter from Laticrete.
Flood Testing a Laticrete Hydro Ban installation.
Does the city of Portland require this step? Does the city of Bridgton require it? Not sure but the maker of the waterproofing product is recommending it - that should speak volumes to you.
I find it surprising the huge resistance that I'm faced in promoting this silly little step. The plumbers in the group here would laugh I'm sure at the concept of skipping this step with a traditional PVC liner and clamping drain. I've seen young plumbers flood out offices before trying to figure out how to seam their membranes. The last big flood out was 12 years ago on Beach Avenue here in Vancouver. That was the last time I let a plumber use a rubber liner on my projects. I work with plumbers every week - not one of them misses doing these shower pans. Not one.
I just emailed Dana and Bruce - both of which work for the government of Maine. I've asked fro some clarification to their plumbing codes and requirements. It seems that Maine follows the UPC and their most recent admendments do not remove the leak test from being preformed.