Avoid ResourceWarnings killing daemons

Newever versions of Python explicitly warn in tests when
resources like files are left hanging around (they'll be closed
eventually).  We were being warned about our termination of the
krb5 daemons.

Additionally, the call to `wait()` could potentially deadlock if
the daemon wrote too much to stdout or stderr.

These issues are fixed by using switching to actually using the
`Popen#terminate` method (instead of calling `kill` in the pid),
followed by `Popen#communicate` (instead of just `Popen#wait`).

k5test/realm.py

@@ -337,8 +337,8 @@ def start_kdc(self, args=[], env=None):
                                             'starting...')
 
     def _stop_daemon(self, proc):
-        os.kill(proc.pid, signal.SIGTERM)
-        proc.wait()
+        proc.terminate()
+        proc.communicate()
         self._daemons.remove(proc)
 
     def stop_kdc(self):