If purchase happens to be installed before MRP is, the "buy" route is
set by default on products *and* takes precedence over the MRP
one (because created first and same sequence).
This leads to purchase trying to buy products (which have no seller)
instead of MRP trying to manufacture them, which is what we're trying
to do & test.
Fix by clearing the route_ids field before adding the MRP routes.
Xavier Morel
346d770291