vecmath: Update to zig 0.16.0, harden tests and add test build step

packages/vecmath/src/vectors/Vector2Int_x8.zig

@@ -171,4 +171,8 @@ pub const Vector2Int_x8 = struct {
     pub inline fn cross(self: Vector2Int_x8, other: Vector2Int_x8) vm.i32x8 {
         return self.x * other.y - self.y * other.x;
     }
+
+    test "refAllDecls" {
+        std.testing.refAllDecls(@This());
+    }
 };