Modify the tests to use memory database backend.

This commit modifies the tests to setup a chain instance backed by the new
memory database backend for btcdb.  This allows the tests to avoid
creating and cleaning up files and also allows the tests to run faster
since it can all happen in memory.

The chainSetup function has also been changed to provide logic to switch
on the database type to allow for easy changing of the backend to a
different database type as needed.  For example, it could be useful to
provide extra testing against new database backends.