test_v2_open_sigs_restart: dont mine blocks until funding tx in mempool

This was flaky because sometimes we'd generate blocks before the funding
transaction reached the mempool.

If we wait until it's in, it works as expected.

tests/test_opening.py

@@ -137,6 +137,9 @@ def test_v2_open_sigs_restart(node_factory, bitcoind):
         l1.rpc.openchannel_signed(chan_id, psbt)
 
     l2.daemon.wait_for_log('Broadcasting funding tx')
+    l2.daemon.wait_for_log('sendrawtx exit 0')
+    txid = l2.rpc.listpeers(l1.info['id'])['peers'][0]['channels'][0]['funding_txid']
+    assert txid in bitcoind.rpc.getrawmempool()
     bitcoind.generate_block(6)
     sync_blockheight(bitcoind, [l1, l2])