Meson: validate appdata without network access
Meson: validate appdata without network access

Currently, we validate the appata file using appstream-util which requires network access to fetch the screenshots and validate them. 
A lot of distros out there run tests without network access, this will make the appdata test fail and they might patch Tilix downstream.
gnunn1/tilix,gnunn1/terminix,gnunn1/tilix,gnunn1/tilix,gnunn1/terminix