Pg testing strategy

As part of the Postgres support #74 !20 , we need to run the same tests we have in SQLite using Postgres as the database backend.

For that, I think the best strategy is:

1. Isolate Autosubmit root dir with the testing cases in two temp directories. One with the .db files for SQLite and the other without them for Postgres.
2. Copy the content of the .db files in the Postgres testing instance.
3. Parametrize a fixture to use both strategies.

These changes are going to be done by just modifying the fixtures and avoiding modifying all the current tests we have.