test: rework lib stuff to share common files

test/lib/emitter/shared/main.cpp

@@ -0,0 +1,21 @@
+#include <gtest/gtest.h>
+#include <entt/core/attribute.h>
+#include "types.h"
+
+ENTT_API void emit(test_emitter &);
+
+TEST(Lib, Emitter) {
+    test_emitter emitter;
+    int value{};
+
+    ASSERT_EQ(value, 0);
+
+    emitter.on<message>([&](message msg, test_emitter &owner) {
+        value = msg.payload;
+        owner.erase<message>();
+    });
+
+    emit(emitter);
+
+    ASSERT_EQ(value, 42);
+}