Howdy, not sure if you are still thinking about a shutter speed tester app. I made myself one that works on a computer that costs only a few bucks and is used in conjunction with audacity free sound recording software. This is accurate to tens of thousands of a second. Not bad for about $15 bucks.
Only thing, is that it is not for the iPhone; however, I have never needed to check my shutter speeds while out in the field making images. I prefer to do a thorough check of all my lenses and then right down the particulars for each lens/shutter combo. My post has a lot of info and photos of the unit.

My post is: Forum
APUG English Forums
Large Format Cameras and Accessories
Shutter Test Log Sheet