fix missing failureException in test_thread

Author: cosminbasca

graalpython/com.oracle.graal.python.test/src/tests/test_thread.py

@@ -279,6 +279,8 @@ def do_finish(self):
 
 
 class BaseTestCase(unittest.TestCase):
+    failureException = AssertionError
+
     def setUp(self):
         self._threads = support.threading_setup()