Fix the stubborn test about cyclic references left by pytest.raises

In Python 2, a context manager's __exit__() leaves sys.exc_info with the exception values even when it was supposed
to suppress the exception, so we explicitly call sys.exc_clear() which removes the traceback and allow the object
to be released.

Also updated the test to not depend on the immediate destruction of the object but instead to ensure it is not being
tracked as a cyclic reference.

Fix #1965

_pytest/python.py

@@ -1237,7 +1237,11 @@ class RaisesContext(object):
                 exc_type, value, traceback = tp
                 tp = exc_type, exc_type(value), traceback
         self.excinfo.__init__(tp)
-        return issubclass(self.excinfo.type, self.expected_exception)
+        suppress_exception = issubclass(self.excinfo.type, self.expected_exception)
+        if sys.version_info[0] == 2 and suppress_exception:
+            sys.exc_clear()
+        return suppress_exception
+
 
 # builtin pytest.approx helper