Bug 15052: Add diagnostic for the TestBuilder tests

Bug 15052: Add diagnostic for the TestBuilder tests

Sometimes t/db_dependent/TestBuilder.t fails with 'TestBuilder can
create a entry for every sources' but we don't know which ones have
failed.

Test plan:
Create the following file: Koha/Schema/Result/Test.pm
 package Koha::Schema::Result::Test;
 use base 'DBIx::Class::Core';
 __PACKAGE__->table("test");
 1;

If you don't have a table named 'test' in your DB,
  prove t/db_dependent/TestBuilder.t
should return a failure, but without any explanation.
With this patch you should get a diag and know what's wrong.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>