Whoops, fix definition of CFFI_BACKEND in tests; led to some test errors on Travis.

src/greentest/greentest/sysinfo.py

@@ -31,7 +31,7 @@ OSX = sys.platform == 'darwin'
 
 # XXX: Formalize this better
 LIBUV = 'libuv' in gevent.core.loop.__module__ # pylint:disable=no-member
-CFFI_BACKEND = bool(os.getenv('GEVENT_CORE_CFFI_ONLY')) or PYPY
+CFFI_BACKEND = PYPY or LIBUV or 'cffi' in os.getenv('GEVENT_LOOP', '')
 
 if '--debug-greentest' in sys.argv:
     sys.argv.remove('--debug-greentest')