Bug 14158: www search tests - Cleanup at the end

Bug 14158: www search tests - Cleanup at the end

When something wrong happen, the parent process is waiting for the
children to finish, but they have been zombified and the test hangs.

To fix that, we should ensure that the cleanup is correctly done and
that the processes are correctly killed.

Test plan:
export KOHA_USER=koha
export KOHA_PASS=koha
export KOHA_INTRANET_URL=http://do_not_exist_but_its_expected
export KOHA_OPAC_URL=http://another_url
prove t/db_dependent/www/search_utf8.t

Without this patch, the tests will hang
With the patch applied, they won't!

Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit 8255a18edc9c2eee52b01bfa20114b088b9bf555)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
(cherry picked from commit 5808b3de5b40bf964813b0c3509f7f7cd0429422)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>