unit definitions: add validation workflow (2/2)

* Add validation workflow to Quantity Unit Conversion Definition & Group
* Only validated objects can define a valid quantity unit
* Add interaction workflow event: when an object is (in)validated,
  invalidate the Cache for site-wide quantities

* Fix b/c test by using proper tear down methods in other tests and by clearing
  the cache


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@32196 20353a03-c40f-0410-a6d1-a30d3c3de9de