If the fixed quantity was higher than the produced quantity, the quantity of the
manufacturing order was used for the byproducts instead of the one specified on
the BoM.
This bug was due to the computation of the subfactor (`_get_subproduct_factor`
in mrp, because I love the modularity) that was computing remaining quantity,
only considering the variable quantities cases.
In fixed quantity, the MO quantity being lower than the byproduct quantity, it
was never fully produced.
Corrected the test to correctly check the quantities and handle the usecase with
an higher quantity in fixed byproduct.
Added a field subproduct_id on the stock.move object that allows to easily find
the attributes of the mrp.subproduct item without making hacky (and probably
wrong in case of more than one byproduct) search to find it back.
Closes#8031
Mitali Patel
7b1d655d8c