Refactor data setup withSeller/Product (bug 938658)

https://bugzilla.mozilla.org/show_bug.cgi?id=938658

The plan is to do that in 2 pass, this is the first one related to `withSeller` and `withProduct`, the second one will be about transaction data (`transData` and `goodTrans`) but I'm waiting for https://github.com/mozilla/zippy/pull/43 to land given that I already did some work in it.