test: shared boxed_int type

test/entt/common/boxed_int.h

@@ -0,0 +1,16 @@
+#ifndef ENTT_COMMON_BOXED_INT_HPP
+#define ENTT_COMMON_BOXED_INT_HPP
+
+namespace test {
+
+struct boxed_int {
+    int value{};
+};
+
+inline bool operator==(const boxed_int &lhs, const boxed_int &rhs) {
+    return lhs.value == rhs.value;
+}
+
+} // namespace test
+
+#endif