[FIX] stock: enforce test to be deterministic

test_putaway_6 creates 2 putaway rules on the same product category but to different location.
As the order on putaway rule is [sequence, product_id] and the two have the same sequence,
it can happens that the choosen rule is not always the same,

Furthermore, the test tried to show the hierarchy between two category matters in term
of putaway rule.

This commit create a child category of 'all' and set it on one of the rule.

closes odoo/odoo#35264

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>