Bug 16407: Fix Koha_borrower_modifications.t

Bug 16407: Fix Koha_borrower_modifications.t

This test was using hardcoded borrower number, assuming they should be
present. Now we use TestBuilder.

Test plan:
Run the test.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Test pass before and after patch.
No errors

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit d8d4277471908bf046d04b4e94eed6cd4c94f63b)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>