pytest: fix test_multifunding_feerates

There's a 1 in 256 chance that our signature on the transaction is 70,
not 71 bytes long.  This changes the freerate.  So fix up the weight in
this case, to be the expected weight.

```
@unittest.skipIf(TEST_NETWORK == 'liquid-regtest', "Fees on elements are different")
@pytest.mark.developer("uses dev-fail")
@pytest.mark.openchannel('v1')  # v2 the weight calculation is off by 3
deftest_multifunding_feerates(node_factory, bitcoind):
'''
    Test feerate parameters for multifundchannel
    '''
        funding_tx_feerate = '10000perkw'
        commitment_tx_feerate_int = 2000
        commitment_tx_feerate = str(commitment_tx_feerate_int) + 'perkw'
        l1, l2, l3 = node_factory.get_nodes(3, opts={'log-level': 'debug'})
        l1.fundwallet(1 << 26)
def_connect_str(node):
return'{}@localhost:{}'.format(node.info['id'], node.port)
        destinations = [{"id": _connect_str(l2), 'amount': 50000}]
        res = l1.rpc.multifundchannel(destinations, feerate=funding_tx_feerate,
                                      commitment_feerate=commitment_tx_feerate)
        entry = bitcoind.rpc.getmempoolentry(res['txid'])
        weight = entry['weight']
        expected_fee = int(funding_tx_feerate[:-5]) * weight // 1000
>       assert expected_fee == entry['fees']['base'] * 10 ** 8
E       AssertionError: assert 7000 == (Decimal('0.00007010') * (10 ** 8))
tests/test_connection.py:1982: AssertionError
```

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

tests/test_connection.py

@@ -1978,6 +1978,9 @@ def _connect_str(node):
     entry = bitcoind.rpc.getmempoolentry(res['txid'])
     weight = entry['weight']
 
+    # If signature is unexpectedly short, we get a spurious failure here!
+    res = bitcoind.rpc.decoderawtransaction(res['tx'])
+    weight += 71 - len(res['vin'][0]['txinwitness'][0]) // 2
     expected_fee = int(funding_tx_feerate[:-5]) * weight // 1000
     assert expected_fee == entry['fees']['base'] * 10 ** 8