zink/ci: add another subtest to the list of known failures for radv

zink/ci: add another subtest to the list of known failures for radv

This test as a whole does not seem to work anywhere, even lavapipe, but
one particular subtest was passing until a recent change
(!19438 - zink: polygon mode fixes?).

After consideration by @kusma, it appears that the subtest was passing
by accident due to zink generating the wrong values. Given that this is
not something that users would ever experience as a regression, we
simply document this new failure along with all the others for this
test.

Fixes: 53721827ea1c ("zink: correct depth-bias enable condition")
Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19517>