Preload songs only after the song list has loaded

At startup and after resetting the playlist, loading the song list and
preloading the first song started at the same time. When the list
answered last, it cleared the playlist and dropped the preloaded song.
Preloading now starts once the list has loaded, and loading the list is
retried if it fails.

tests/fakes/main_window_test_suite.gd

@@ -36,12 +36,19 @@ func before_test() -> void:
     _server.respond_bytes("/music/reset", PackedByteArray())
     _server.respond_bytes("/music/played?song=0", PackedByteArray())
 
+    _before_window_starts()
     _runner = scene_runner("res://MainWindow.tscn")
     _window = _runner.scene()
     # The debug build lowers the winning score on startup; keep it out of reach
     Settings.winning_score = 20
 
 
+## Override to change the fake server before the window starts and sends its first
+## requests.
+func _before_window_starts() -> void:
+    pass
+
+
 func after_test() -> void:
     # Let nodes queued for deletion by the window be freed before orphans are counted
     await await_idle_frame()