Use the database_cleaner gem to handle database persistence during tests.
This is needed because SuckerPunch jobs are executed from a dedicated
thread, for both synchronous and asnychronous execution. Hence, the data set
from the MiniTest fixtures is not visible from the other thread as their
removal races against the task performer.

To fix this, set DatabaseCleaner.strategy to :truncation and implement
ActiveSupport::TestCase::setup() and ::teardown().

Also set SuckerPunch to synchronous execution for all test cases.