Need high-level compatibility test suite

[riannucci]

Basically there are some tests in impl/* which should probably move to service/*/compat_tests. This would allow all implementations (except dummy...) to just import them and we could make sure that there's cohesion between memory and prod.

Since the prod tests (with aetest) are so freaking slow (like 3-4s overhead per test)... I would only expect to run them by hand, or /maybe/ on a CI waterfall somewhere (not as blocker tests). But being able to run all the high-level tests under an arbitrary implementation would be pretty sweet.

If we add another level of cleverness we could even have it generate an appengine stub, deploy it, run the tests (for real in production) and collect them back to get compatibility data from teh klouds.