The idea here is that interface_test.go will be directly usable in each
implementation to increase test coverage there as well, but also works at
the top-most level to test arbitrary backends.
This is simply at start at providing generic interface tests. The only
thing is tests so far is the empty database conditions on NewestSha, but
it adds infrastructure for creating and opening databases with special
type handling based on the database and necessary logic to teardown so
multiple backends can be tested simultaneously.
This and the next series of commits all discussed with drahn@.
