Use pytest fixture for the orasql test.
Remove unused import
Fix a bug in the script ``oradiff`` that resulting in the wrong order of the output.

The output order was 1) drop objects only in db1, 2) create objects that differ,
3) create object only in db2, so if an existing table got a new foreign key
to a new table, the order was wrong.

Now the order is: 1) (re)create objects that differ or are only in db2, 2) drop
objects only in db1.