anv/tests: Zero-initialize instances

Some of the tests were actually relying on some of those uninitialized
bits to be non-zero.  In particular, a couple want use_softpin = true.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit 9076e9f3)