A typo, `annotator` instead of `@annotator`, caused this functionality
to break when `window.annotator` (which was the same object as
`@annotator`) was removed in 0033f774.

Correct the typo and add tests. The tests operate at the DOM level so
they should be easy to adapt if the BucketBar implementation is
rewritten or changed substantially in future.

Fixes https://github.com/hypothesis/product-backlog/issues/704