tools/bench-gossipd.sh: make sure we wait for gossipd to startup sync.

It can take a while if bitcoind has the regtest chain, and grossly
distorts our benchmarks!

Reported-by: Joe Netti <jnetti@blockstream.io>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/bench-gossipd.sh

@@ -23,6 +23,8 @@ wait_for_start()
 	    exit 1
 	fi
     done
+    # Wait for it to catch up with bitcoind.
+    while [ "$($LCLI1 -H getinfo 2>/dev/null | grep '^blockheight=' | cut -d= -f2)" != "$(bitcoin-cli -regtest getblockcount)" ]; do sleep 1; done
     echo "$ID"
 }