Bug 27636: (QA follow-up) Fix tests and validate

Bug 27636: (QA follow-up) Fix tests and validate

This patch fixes the tests for when a negative amount is passed to the
pay method.  Prior to now, a negative amount would have been passed
through and recorded. This was inconsistent with all other accounts
methods and has been deprecated to ensure consistent amounts handling.

This patch also introduces basic validation to prevent negatives being
entered into the UI.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>