pytest: fix flaky test if one side hasn't processed close yet.

```
    @unittest.skipIf(TEST_NETWORK != 'regtest', "External wallet support doesn't work with elements yet.")
    def test_funding_close_upfront(node_factory, bitcoind):
...

        # check that you can provide a closing address upfront
        addr = l1.rpc.newaddr()['bech32']
>       _fundchannel(l1, l2, amt_normal, addr)
...
pyln.client.lightning.RpcError: RPC call failed: method: fundchannel_start, payload: {'id': '022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59', 'amount': 100000, 'announce': True, 'close_to': 'bcrt1qctx2k9cu9fd7nk449mzphqjcvvpyc4rxh6826x'}, error: {'code': -1, 'message': 'They sent error channel 2a1ca624cd1127761cb7a4395df2c3fd6d0abb3732c1f85a5345b0da716540d0: Multiple channels unsupported'}
```

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

tests/test_connection.py

@@ -1080,10 +1080,19 @@ def test_funding_close_upfront(node_factory, bitcoind):
 
     remote_valid_addr = 'bcrt1q7gtnxmlaly9vklvmfj06amfdef3rtnrdazdsvw'
 
+    def has_normal_channels(l1, l2):
+        return any([c['state'] == 'CHANNELD_AWAITING_LOCKIN'
+                    or c['state'] == 'CHANNELD_NORMAL'
+                    for c in only_one(l1.rpc.listpeers(l2.info['id'])['peers'])['channels']])
+
     def _fundchannel(l1, l2, amount, close_to):
         l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
         assert(l1.rpc.listpeers()['peers'][0]['id'] == l2.info['id'])
 
+        # Make sure both consider any previous channels closed.
+        wait_for(lambda: not has_normal_channels(l1, l2))
+        wait_for(lambda: not has_normal_channels(l2, l1))
+
         resp = l1.rpc.fundchannel_start(l2.info['id'], amount, close_to=close_to)
         address = resp['funding_address']