add and fix a number of cpython yield_from tests

Author: timfel

graalpython/com.oracle.graal.python.test/src/graalpytest.py

@@ -175,6 +175,9 @@ def assertIsInstance(self, obj, cls, msg=None):
                 message = '%s is not an instance of %r' % (obj, cls)
             assert False, message
 
+    def fail(self, msg):
+        assert False, msg
+
     class assertRaises():
         def __init__(self, exc_type, function=None, *args, **kwargs):
             self.function = function