test: rework lib stuff to share common files

test/lib/emitter/shared/lib.cpp

@@ -0,0 +1,8 @@
+#include <entt/core/attribute.h>
+#include "types.h"
+
+ENTT_API void emit(test_emitter &emitter) {
+    emitter.publish(event{});
+    emitter.publish(message{42});
+    emitter.publish(message{3});
+}