This diff introduces a test plan for the `removeMessageItemsOlderThenDeviceCheckpoint()` database method that was introduced in D4220.
The test flow is following:
1. Insert new message item.
2. Find the message item by the receiver deviceID.
3. Check if found items are not zero.
4. Call `removeMessageItemsOlderThenDeviceCheckpoint` with the deviceID and checkpoint time greater than that was during the first insert.
5. Find the message item by the receiver deviceID.
6. Check if the found items are equal to zero.