Fix FakeAkonadiServer-based unit-tests deadlocking

This was caused by the thread management change in d9fb479d which
moved Connection to it's own QThread. As a result FakeAkonadiServer
calling FakeConnection::notificationCollector() caused m_backend
to be initialized to caller-thread DataStore instead of Connection-
thread DataStore.

As a result methods using Connection::dataStore() and DataStore::self()
got a different QSqlDatabase, causing transaction deadlock.