make trac.ticket.tests.functional a directory so that we can start splitting up the tests into submodules organized by topic
make a new trac/ticket/tests/functional/default_workflow.py suite, and move the may_set_owner tests there; also adding a few new may_set_owner tests
add another functional test
make code a bit more self-documenting and less likely to fail in the future if an additional set-owner-ish operation is added to the outer if statement