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

packages/vecmath/src/vectors/Vector3Int_x8.zig

@@ -180,4 +180,8 @@ pub const Vector3Int_x8 = struct {
             .z = self.x * other.y - self.y * other.x,
         };
     }
+
+    test "refAllDecls" {
+        std.testing.refAllDecls(@This());
+    }
 };