autotests: remove code duplication by moving try+catch+qWarning+qFatal to FakeAkonadiServer

Summary: Less code to write in new unittests is always a good thing

Test Plan:
ctest, all passed except akonadi-sqlite-referencetest, which is unrelated
and on death row anyway.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20355