Bug 25527: Add logger to Koha::ExternalContent

Bug 25527: Add logger to Koha::ExternalContent

This patch makes Koha::Logger initialization happen in the ->new method
for the Koha::ExternalContent-derived classes. In the case of RecordedBooks,
it doesn't look like it is used at all.

In the case of OverDrive, it will now use the Koha::ExternalContent
exported logger accessor.

I added tests for this addition to Koha::ExternalContent to the
OverDrive tests.

I also removed references to Test::DBIx::Class as it is not used at all.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha_ExternalContent_OverDrive.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>