pyloxi: add basic test cases for of11 and of12

Also fixes minor bugs when importing those modules, and adds a check-py target
to run all the unit tests against the generated code.
diff --git a/Makefile b/Makefile
index 6b63fa6..e77edfe 100644
--- a/Makefile
+++ b/Makefile
@@ -72,6 +72,12 @@
 check:
 	PYTHONPATH=. ./utest/test_parser.py
 
+check-py: python
+	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/of10.py
+	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/of11.py
+	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/of12.py
+	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/of13.py
+
 pylint:
 	pylint -E ${LOXI_PY_FILES}