stackinit: Add old-style zero-init syntax to struct tests

The deprecated way to do a full zero init of a structure is with "= { 0 }",
but we weren't testing this style. Add it.

Link: https://lore.kernel.org/r/20250127191031.245214-1-kees@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>