Break out compute API unit tests

As we convert things to objects, it'll be easier to convert our compute
API tests to be true unit tests if they are in their own class.  This
also allows us to move away from the hacks needed to simulate 2 DBs in
order to test ComputeCellsAPI().

There are currently a handful of tests that are true unit tests and
require no DB access.  These are now moved, subclassing from
test.NoDBTestCase.

Related to blueprint unified-object-model

Change-Id: I72f4f6ada8d0586e813e551a1b361bcebb16ee66
Merge "Break out compute API unit tests"