For more than a year now, we have started to write tests using the unit testing library unittest2. The unittest2 tests complement the YAML-based tests when we need more control on the way tests are executed, or we need to write more Python code than data, for which YAML provide a simpler syntax.
As we have expanded on the unit test infrastructure, the OpenERP trunk branches now contain some specific unittest2 base classes to help write tests. They also contain Javascript unit tests using the qunit library.
Python and Javascript unit testing in OpenERP