Add failing test to show rounding problem

Hello,

This PR adds unittest for issue #2072.

Scenario to reproduce the problem by hand:
1/ Set 120 quantity on a product
2/ Sell a dozen(uom) of product
3/ Confirm and deliver product
4/ The Quantity On Hand is 107 !! instead of 108

The bug seems to be there from a long time.
* https://www.odoo.com/forum/help-1/question/unit-of-measure-rounding-does-not-allow-you-to-ship-error-with-my-uom-53988
* https://code.launchpad.net/~openerp-dev/openobject-addons/7.0-opw-601528-rgo/+merge/200212
* ...