fda63b4 unfortunately broke highlighting in PDFs, because the PDF
anchoring code first finds the text in the document, and then uses a
TextPositionSelector and the HTML anchoring code to select a range in
the rendered page for highlighting.

This commit fixes that issue by moving the TextQuoteSelector check up a
level into `Guest#anchor`.

I've added two tests for this in `guest-test.coffee` -- one tests that
annotations where the target contains no TextQuoteSelector are marked as
orphans, and the second tests that the low-level anchoring code is never
called in this case.

I have also removed the integration test, on the basis that I don't
think integration testing failure cases is a good habit to get into.

Fixes #3530.