Remove unused Config#redis_prefix_ryespy , tweaking tests.
Test Notifier::Sidekiq,Listener::FTP,Listener::IMAP .

Add `Test::config`,`Test::Redis` helpers. Tests are run against a live
Redis service (`REDIS_URL`) with a random namespace, with keys getting
cleaned after each test. It shouldn't be necessary to use a separate
db since keys are removed within the namespace, rather than the db being
flushed.

Rework `Notifier::Sidekiq#notify` tests, checking that keys are set as
expected. Test `Listener::FTP#check`, checking that files are detected
and notified. Test `Listener::IMAP#check`, checking that emails are
detected and notified. Tests for `FTP`,`IMAP` are not run against live
services.