CollectionScheduler: add tests for the actual QTimer interval

Summary:
Testing the data structures is all fine and good, but I wanted to
make sure the actual underlying timer was set to the right interval,
after all that std::chrono wrangling.

Test Plan: collectionschedulertest still passes

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D20483