add xtest_signal.py which reproduces SIGCHLD being missed (fixed in https://bitbucket.org/denis/gevent/changeset/adb8b5ac698c)

greentest.py: in dbg mode, call setUp() and tearDown() properly when executing tests multiple times for refcount

better start up procedure: check that we started by connecting to an app; also use this check to make sure no other app is bound to this socket