Test that sqlite DatabaseDS server can launch

The DatabaseDS application in tango/databaseds/database.py should have a basic test. We have had a few issues during release candidate testing where obvious things have not been updated.

We need to exercise the main() function in the test, so it could be launched with something like:

python -m tango.databaseds.database --port=30000 --logging_level=2 2

Then connect with a DeviceProxy and test at least one command.

In the related issues there are discussions about the future of this databaseds, and the possibility of moving it into a dedicated repo. That may still happen, but in the meantime a simple test would be beneficial. Some fixes from the related repo, pytango-db, need to be backported.

Related: #414 (closed) and #266 (closed)