Bug 19130: (followup) Add t/db_dependent/Koha/Acquisition/Booksellers.t

Bug 19130: (followup) Add t/db_dependent/Koha/Acquisition/Booksellers.t

This followup patch adds a proper file in which add tests for
Koha::Acquisition::Bookseller(s) methods.

All current methods are covered.

To test:
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/db_dependent/Koha/Acquisition/Booksellers.t
 => SUCCESS: Test pass!
 - Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>