Test that the song cache fills up to the cache size setting

Covers a cache size above 1: songs are preloaded until the setting is
reached, playing a song tops the cache up again, and the cache keeps
filling after a failed download once the server answers again.

tests/integration/test_main_window.gd

@@ -119,3 +119,42 @@ func test_failed_song_info_retries_only_that_step() -> void:
     assert_int(Playlist.song_object_array.size()).is_equal(songs_before + 1)
     assert_str(Playlist.song_object_array.back().song_title).is_equal("Dancing Mad")
     assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func test_preloads_as_many_songs_as_the_cache_setting() -> void:
+    Playlist.number_of_tracks_to_preload = 3
+
+    _runner.invoke("fetch_song")
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+    assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func test_playing_a_song_tops_the_cache_up_again() -> void:
+    Playlist.number_of_tracks_to_preload = 3
+    _runner.invoke("fetch_song")
+
+    await _play_first_song()
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+    assert_int(Playlist.song_object_array.size()).is_equal(4)
+
+
+func test_cache_keeps_filling_after_a_failed_download() -> void:
+    _window.song_fetch_retry_seconds = 0.05
+    Playlist.number_of_tracks_to_preload = 3
+    var mp3: PackedByteArray = FileAccess.get_file_as_bytes("res://tests/data/short.mp3")
+    # One more download works, then the server stops answering
+    _server.fail_after("/music/rand", 1)
+
+    _runner.invoke("fetch_song")
+    await await_millis(150)
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(2)
+    assert_bool(Playlist.is_song_downloading).is_true()
+
+    _server.respond_bytes("/music/rand", mp3)
+    await await_millis(150)
+
+    assert_int(Playlist.get_preloaded_songs()).is_equal(3)
+    assert_bool(Playlist.is_song_downloading).is_false()