Bug 19049: [QA Follow-up] Mock config, default format

Bug 19049: [QA Follow-up] Mock config, default format

As requested by QA:
[1] Mock_config enable_plugins in the test.
[2] Fallback to MARC when format is empty. Remove die statement.
Added:
[3] Remove $marc. This variable got obsolete during development.
[4] Add test on $input_file and $plugin_class. Test $text before calling
    Handler or processing $text. No need to split undef if somehow Handler
    returned undef, etc. If the routine returns an empty arrayref,
    stage-marc-import will do fine.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>