Fix checkBlockCompletion parameters

We were assuming a prop named unitId existed in CoursewareContainer - it doesn’t.  unitId is not in redux.  What we do have, is the unitId in the route params - what we refer to as routeUnitId.  If we use this instead of the non-existent unitId, then life is good.

I wrote a test (that breaks!) prior to implementing the fix.  The fix satisfies the test. 🎉

src/courseware/course/sequence/sequence-navigation/UnitNavigation.jsx

@@ -60,7 +60,11 @@ export default function UnitNavigation(props) {
 
 UnitNavigation.propTypes = {
   sequenceId: PropTypes.string.isRequired,
-  unitId: PropTypes.string.isRequired,
+  unitId: PropTypes.string,
   onClickPrevious: PropTypes.func.isRequired,
   onClickNext: PropTypes.func.isRequired,
 };
+
+UnitNavigation.defaultProps = {
+  unitId: null,
+};