Restore DB connections for every :use_database spec group. #3936.

The prior behavior assumed that no other specs ever called
`DatabaseInitializer#set_up_repository`. There are some that do, it's
just that prior to 70c59ff9 all the :use_database specs happened after
the specs that called `#set_up_repository` with a different
connection. This made it so that this interference wasn't noticeable.
Once a :use_database spec was added before a spec which called
`#set_up_repository` (contents_spec before transform_load_spec), this
became a problem when running the full spec suite.