pytest: note unfixable test_htlc_rexmit_while_closing test.

We really need our own lnprototest tests for packet-based stuff;
these message-based tests are inherently delicate and awkward.

In particular, connectd now does dev-disconnect, so the socket is not
immediately closed after a dev-disconnect command.  In this case, the
WIRE_SHUTDOWN has often already been written from connectd to channeld.

But it sometimes works, too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_closing.py

@@ -3438,6 +3438,7 @@ def test_closing_higherfee(node_factory, bitcoind, executor):
 @pytest.mark.developer("needs dev_disconnect")
 def test_htlc_rexmit_while_closing(node_factory, executor):
     """Retranmitting an HTLC revocation while shutting down should work"""
+    # FIXME: This should be in lnprototest!  UNRELIABLE.
     # l1 disconnects after sending second COMMITMENT_SIGNED.
     # Then it stops receiving after sending WIRE_SHUTDOWN (which is before it
     # reads the revoke_and_ack).