Bug 20311: Prevent get_age tests to fail on Feb 28th

Bug 20311: Prevent get_age tests to fail on Feb 28th

Today Feb 28th 2018:
If we subtract 18y to this DateTime object 29/02/2000 and so the patron is not major yet

use Koha::DateUtils qw( dt_from_string );
say dt_from_string('28/02/2018')->add(years => -18);
say dt_from_string('28/02/2018')->add(years => -18, end_of_month => 'limit');

will display:
2000-02-29T00:00:00
2000-02-28T00:00:00

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