run_tests.sh now reports success/failures by exit code

Signed-off-by: Vinzenz Feenstra <evilissimo@gmail.com>
Multiple issues solved

* None is no longer threated like a keyword
* future imports checks are only raising errors now when
  'handle_future_imports' is enabled
* Nested 'AstSuite' elements are flattened. (Resulted from multi statement
  lines, e.g. print foo; print bar; yield y;

Signed-off-by: Vinzenz Feenstra <evilissimo@gmail.com>