Properly cleanup in notificationmanagertest

Summary:
The call to "quit" is symmetrical with

QMetaObject::invokeMethod(&manager, "init", Qt::DirectConnection);

on line 56 of the test (due to NoThread mode).

But for the call to quit() to work, the base class has to not assert.

Test Plan: bin/notificationmanagertest in an ASAN build

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D21132