FakeCollectionScheduler => FakeIntervalCheck, so FakeAkonadiServer can return it

Summary:
This way AkonadiServer::instance()->intervalChecker() is never null
anymore, which simplifies some code at little bit, and will allow
testing how NotificationCollector interacts with IntervalCheck.

BTW CacheCleaner needs the if(), it's optional (null if configured away by user)

Test Plan: full "ctest"

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20512