(#6771) Disconnect any ActiveRecord connection after every test.

Some of our tests can trigger an actual ActiveRecord connection being made.
This could leak between tests, potentially causing odd problems, or otherwise
share state between them.

We should, when we have the feature available, ensure we disconnect between
every test, so that we don't share state.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>