Test server test passing.

* tests/test_utils.py:
(full_scenarios): Complete muddleftpd definition.

* test_server.py:
(MuddleftpdFeature, Muddleftpd): New classes.
(get_test_permutations): Add muddleftpd permutation.

* server.py:
(Server._kill_server): Add docstring, return True if zombie
catched.
(Muddleftpd.is_installed): We need ftpwho too.
(Muddleftpd.get_pid): 'ftpwho -p' is the reliable way to get the
pid.
(Muddleftpd._start): Use get_pid() to ensure the server was started.
(Muddleftpd._stop): There is no pidfile.

* configs/muddleftpd.conf:
(scratchfile): Mention that this more or less implies that we run
a single server only.