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

packages/vecmath/src/vectors/Vector3x8.zig

@@ -258,4 +258,8 @@ pub const Vector3x8 = struct {
             .z = v.x * vm.ps(self.iz) + v.y * vm.ps(self.jz) + v.z * vm.ps(self.kz),
         };
     }
+
+    test "refAllDecls" {
+        std.testing.refAllDecls(@This());
+    }
 };