testing: fix test_closing_higherfee

shutdown_subdaemons frees the channel and calls destroy_close_command_on_channel_destroy, see gdb:

    0  destroy_close_command_on_channel_destroy (_=0x55db6ca38e18, cc=0x55db6ca43338) at lightningd/closing_control.c:94
    1  0x000055db6a8181b5 in notify (ctx=0x55db6ca38df0, type=TAL_NOTIFY_FREE, info=0x55db6ca38e18, saved_errno=0) at ccan/ccan/tal/tal.c:237
    2  0x000055db6a8186bb in del_tree (t=0x55db6ca38df0, orig=0x55db6ca38e18, saved_errno=0) at ccan/ccan/tal/tal.c:402
    3  0x000055db6a818a47 in tal_free (ctx=0x55db6ca38e18) at ccan/ccan/tal/tal.c:486
    4  0x000055db6a73fffa in shutdown_subdaemons (ld=0x55db6c8b4ca8) at lightningd/lightningd.c:543
    5  0x000055db6a741098 in main (argc=21, argv=0x7ffffa3e8048) at lightningd/lightningd.c:1192

Before this PR, there was no io_loop after shutdown_subdaemons and client side raised a
general `Connection to RPC server lost.`
Now we test the more specific `Channel forgotten before proper close.`, which is good!

BTW, this test was added recently in PR ElementsProject#4599.

tests/test_closing.py

@@ -3099,7 +3099,7 @@ def test_closing_higherfee(node_factory, bitcoind, executor):
     l1.daemon.wait_for_log(r'deriving max fee from rate 30000 -> 16440sat \(not 1000000sat\)')
 
     # This will fail because l1 restarted!
-    with pytest.raises(RpcError, match=r'Connection to RPC server lost.'):
+    with pytest.raises(RpcError, match=r'Channel forgotten before proper close.'):
         fut.result(TIMEOUT)
 
     # But we still complete negotiation!