feat(test|api): add AlbumSong tests

tests/Feature/V6/AlbumSongTest.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Tests\Feature\V6;
+
+use App\Models\Album;
+use App\Models\Song;
+
+class AlbumSongTest extends TestCase
+{
+    private const JSON_STRUCTURE = [
+        'type',
+        'id',
+        'title',
+        'lyrics',
+        'album_id',
+        'album_name',
+        'artist_id',
+        'artist_name',
+        'album_artist_id',
+        'album_artist_name',
+        'album_cover',
+        'length',
+        'liked',
+        'play_count',
+        'track',
+        'disc',
+        'created_at',
+    ];
+
+    private const JSON_COLLECTION_STRUCTURE = [
+        '*' => self::JSON_STRUCTURE,
+    ];
+
+    public function testIndex(): void
+    {
+        /** @var Album $album */
+        $album = Album::factory()->create();
+
+        Song::factory(5)->create([
+            'album_id' => $album->id,
+        ]);
+
+        $this->getAsUser('api/albums/' . $album->id . '/songs')
+            ->assertJsonStructure(self::JSON_COLLECTION_STRUCTURE);
+    }
+}