Bug 30181: Make Koha::BackgroundJob->_derived_class return the right thing

Bug 30181: Make Koha::BackgroundJob->_derived_class return the right thing

This patch makes _derived_class rely on _new_from_dbic to generate a new
object with the right class, but based on the same DB row. Not an empty
one as it was before. This way we can remove some biolerplate that is
required now when writing background job classes.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/BackgroundJob.t
=> FAIL: Boo, tests fail
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>