Bug 20997: Add unit tests for Koha::Account::Line::apply

Bug 20997: Add unit tests for Koha::Account::Line::apply

This patch adds unit tests for the mentioned method.

To test:
- Apply this patch
- Run:
  $ khell
 k$ prove t/db_dependent/Koha/Account/Lines.t
=> FAIL: The method is not already implemented!

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>