Add missing __test__ check for test discovery.

_pytest/python.py

@@ -503,6 +503,8 @@ def _get_xunit_func(obj, name):
 class Class(PyCollector):
     """ Collector for test methods. """
     def collect(self):
+        if not safe_getattr(self.obj, "__test__", True):
+            return []
         if hasinit(self.obj):
             self.warn("C1", "cannot collect test class %r because it has a "
                 "__init__ constructor" % self.obj.__name__)