Acceptance Tests: bump Avocado version requirement to 85.0

Acceptance Tests: bump Avocado version requirement to 85.0

This version (and 84.0) contain improvements that address specific
QEMU use cases, including:

 * Being able to download and use Fedora 31 images and thus
   re-activate the "boot_linux.py" tests

 * Being able to register local assets via "avocado assets register"
   and use them in tests

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210211232835.2608059-2-crosa@redhat.com>
Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>