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.

Linear task: ENG-1303

Run test-tunnelbroker-service-dev-mode, successfully passed RemoveMessageItemsOlderThenCheckpoint test.