As noted in the comments in source4/selftest/tests.py:
# Demote the vampire DC, it must currently be the last test on the VAMPIRE DC
This should not be the case, as it prevents us from running tests in random order.
There are a couple of ways we can fix this:
* mark the test as 'dirty' so that selftest.pl knows to discard the environment and set it up again from scratch
* have samba_tool_demote do a promote after it has done the demotion