app-editors/ghex: ebuild touchups, EAPI-7 bumps, restrict tests

app-editors/ghex: ebuild touchups, EAPI-7 bumps, restrict tests

* Bump to EAPI-7
* Downgrade yelp-tools dep to correct itstool dep
* Ensure bug-free ITS supporting gettext
* Depend on libxml2 for xml-stripblanks gresource usage
* Depend on appstream-glib for appdata translation due to tags
  used not present in gettext own provided appdata.its
* Restrict tests, as upstream thinks it's great to depend on
  an unreleased appstream-glib for tests to succeed

Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>