Check doc-cover for the cover image

Please provide the following details if possible or relevant:

- [1.1.1] the version of Ace impacted by the issue (as returned by the `ace -v` command).
- [] the version of Node used to run Ace (as returned by the `node -v` command).
- [] your operating system and version.
- [] if the issue is a bug when running Ace: the error message as produced in the log file (you can enable debug logging with the `--verbose` option).
- [] if the issue is about the HTML report: the browser(s) and version(s) you used.
- [] if possible, let us know if you can share a sample input file to help us reproduce the issue.

Following the mapping in "EPUB Type to ARIA Role Authoring Guide", we have to put a role doc-cover for the EPUB cover image.
That image is declared explicitly in the manifest with the property "cover-image".
Ace should verify that the img tag for that specific image has the role "doc-cover" and report a violation if not.