Add JAX PPO opponent to classic GUI

tests/games/classic/test_pygame_pvp.py

@@ -18,6 +18,8 @@ def test_gui_argparser_accepts_classic_options() -> None:
             "1024",
             "--height",
             "768",
+            "--jax-checkpoint",
+            "/tmp/final_candidate",
         ]
     )
 
@@ -26,6 +28,7 @@ def test_gui_argparser_accepts_classic_options() -> None:
     assert args.seed == 7
     assert args.width == 1024
     assert args.height == 768
+    assert args.jax_checkpoint == "/tmp/final_candidate"
 
 
 def test_gui_argparser_rejects_removed_backend_option() -> None: