Bug 25670: Fix random failure on Orders.t

Bug 25670: Fix random failure on Orders.t

It has failed at least 3 times:

 #   Failed test 'The right order is returned'
 #   at t/db_dependent/Koha/Acquisition/Orders.t line 153.
 #          got: '50'
 #     expected: '49'

 #   Failed test 'The right order is returned'
 #   at t/db_dependent/Koha/Acquisition/Orders.t line 154.
 #          got: '49'
 #     expected: '50'
 # Looks like you failed 2 tests of 6.

I cannot manage to make it fail locally

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>