fix up 22763d86: the workflow fixture must not have scope 'session' as this way it will only get loaded once (but still torn down via the 'config' fixture)

note that for the default content to be correct (in particular the 'root' node) the workflow must be loaded before its creation
the ACL for the root node has been split up due to the now active workflow (see 22763d86)
remove unused & unneeded fixtures
use `root` fixture instead of `get_root` for testing
let's also use the `db_session` fixture (instead of `DBSession`)
revert 85db1ca5 re-enabling the alembic-related test

note that in order to prevent a complete teardown and rebuild of the db structure we mock the list of tables known to the db engine.  this way the stamp is created without affecting other tests...
Merge pull request #3 from witsch/get_rid_of_unittest

getting rid of unittest, part 2