[FIX] account: fiscal position computation precedence

Zip range and state filtering were added at
377a6eca, but the
precedence between country, group, vat_required,
and not required were not correct.

This commit fixes the precedence as follows and adds
the corresponding tests:

 0. Explict position on partner
 1. Direct match on all criterions
 2. Match on country+zip before country+state
 3. Match on country before country group
 4. Match on `vat_required` criterion on all
    the above before trying them all again without