Add pos arg to Tree.next()
parse_test.go: Remove panics
parse_test.go: Add checkMatchingFields()

Compares the parser output nodes to the expected nodes making sure each has the
appropriate fields. If a field is missing in either of the outputs an error is
generated.
Add MessageType field to NodeSystemMessage
Add missing length to OverlineBad0500

Thanks to checkMatchingFields() this missing length was discovered.