Add back-compat for old-style sentry indexing

[tvansteenburgh]

When a test does sentry['mycharm/0'], do sentry['mycharm'][0] behind-the-scenes instead of returning a KeyError. I.e., use the unit num as an index into the sentry array. This will make old tests work again without having to update them all. Arguably this is what we should have done in the first place.

[johnsca]

+1