Fix FakeAkonadiServerCommands being leaked

Summary:
Porting to unique_ptr would be complex here because each command
goes through the following containers, between creation and usage+deletion:
- QList<FakeJobResponse *>
- QList<FakeAkonadiServerCommand *>
- QQueue<FakeAkonadiServerCommand *>

Test Plan: tagmodeltest with ASAN (but it has other leaks)

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D21139