Add note to memleak tests

tests/test_regression.py

@@ -52,6 +52,8 @@ class _NoLeakAsserter(object):
         gc.enable()
 
 
+# XXX: untitaker: These tests need to be revised. They broke around the time we
+# ported Flask to Python 3.
 @pytest.mark.skipif(os.environ.get('RUN_FLASK_MEMORY_TESTS') != '1',
                     reason='Turned off due to envvar.')
 class TestMemory(object):