Update CLI script integration tests

This PR:
- Refactors `test_script.py` to us `CLITest` abstract class defined in #1688
- Updates existing integration tests for `bin/omero script` subcommands
- Adds an integration test for `bin/omero script demo` (see https://trac.openmicroscopy.org.uk/ome/ticket/4335)

To test this PR, check the integration tests are green in the Test Results of the OMERO-integration-develop job