node tests: Bring list_cursor to 100% coverage.

We had some indirect coverage of this already, so
this mostly focuses on error cases and corner
cases.

tools/test-js-with-node

@@ -45,6 +45,7 @@ enforce_fully_covered = {
     'static/js/filter.js',
     'static/js/hash_util.js',
     'static/js/keydown_util.js',
+    'static/js/list_cursor.js',
     'static/js/markdown.js',
     'static/js/message_store.js',
     'static/js/muting.js',