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

packages/vecmath/src/matrices/Matrix4x4.zig

@@ -507,4 +507,8 @@ pub const Matrix4x4 = extern struct {
             // zig fmt: on
         };
     }
+
+    test "refAllDecls" {
+        std.testing.refAllDecls(@This());
+    }
 };