Integration test Que working jobs
Integration test Que working jobs

This change introduces end-to-end testing of Que workers pulling and
working jobs from a queue, in parallel. This serves as confirmation in
the previous refactor changes, and offers some compensation for the
temporary removal of the legacy_specs.

Co-authored-by: Harry Maclean <02c796074727072587e45a895f8b49f2b5141517@gocardless.com>

gocardless/que,gocardless/que