Fixes#202.
Test expectations are updated. For example, BUFFER='/bin' is now
highlighted as path_prefix because it's a prefix of '/bin/sh' which
would be valid. However, BUFFER='/bin;' is now properly highlighted
as an error (unless AUTO_CD is set).
This is a followup to the last commit, which was about issue #328.
These tests are taken from the original #328 pull request, with updated
expectations where applicable.
Review-by: Matthew Martin