external.py: Adds ReadWriteSelectListDbBase() test to test suite

dbbase.py: Removes read(), write(), list(), and select() doctests
external.py: Adds OverlayObjTest() test to test suite

overlay.py: Removes get_infostr(), short_list(), and __init__()
doctests.
external.py: Adds RemoteDBCache() test to test suite

remotedb.py: Removes cache() doctest.
external.py: Adds MakeOverlayXML test to test suite