Akonadi: fix timing-dependent failure of partstreamertest

Summary:
The deletion happens in another thread
(in commit() called by Akonadi::Server::Connection::handleIncomingData)
so we might have to wait a little bit in the test.

Test Plan: ctest -j8

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D18889