Akonadi tests: make checkTestIsIsolated work in release mode too

Summary: Unit tests wouldn't abort when run directly, otherwise

Test Plan: bin/etmcalendartest in akonadi-calendar, now aborts.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12383