Split tests up by tested module

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Test for missing start time in inverter config

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Test for attempting to open an empty DB

Fix a bug in the db module thereby found

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>