pyloxi: start using data files for test cases

For the moment I'm just replacing the test functions one by one instead of
iterating over all test data files. We should think about doing that but I'm
not sure how to make failures easy to debug.
diff --git a/Makefile b/Makefile
index efc50dc..fb75c08 100644
--- a/Makefile
+++ b/Makefile
@@ -82,11 +82,11 @@
 	PYTHONPATH=. ./utest/test_test_data.py
 
 check-py: python
-	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/generic_util.py
-	PYTHONPATH=${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=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/of13.py
+	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi:. python py_gen/tests/generic_util.py
+	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}