In the upcoming migration of the firewall database to SQL, the helper
functions that creates the test databases of different types, need to
return a unified interface in order to not have to control the
migration tests file by build tags. Therefore, we update the
`NewTestDB` functions to return the `FirewallDBs` interface instead of
the specific store implementation type.
We add a helper function to the functions that creates the test SQL
stores, in order to ensure that the store is properly closed when the
test is cleaned up.
In preparation for using the clock to get an Action's AttemptedAt time
in an upcoming commit, we let both the bbolt and SQL impls of the
firewalldb take a clock.
In this commit, we let the SQLDB implement the RulesDB interface. Unit
tests are updated such that they can be run against sqlite and postgres
DBs using the SQLDB.
