Re-enable non-Akonadi tests in messagelib and mailcommon

We still haven't figured out why the Akonadi testrunner doesn't exist properly
sometimes on the CI so keep those disabled, but we can at least enable the
regular non-Akonadi tests in those libraries.

CCMAIL: kde-pim@kde.org

Differential Revision: https://phabricator.kde.org/D22944