Bug 18286: [Follow-up] Remove assumption on branch count

Bug 18286: [Follow-up] Remove assumption on branch count

A test in db_dependent should not make assumptions on the number of
branches in the database. If you need one, create one. Removing the
assumption of a non-zero count.

Removing the library count statement outside the subtest.
Replacing C4::Context by Koha::Database.

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