itest: fix sign_psbt failure

Ensure the wallet has synced the blockchain before attempting to spend
funds.

Prior to this fix, I get the following error:
  rpc error: code = Unknown desc = wallet couldn't fund PSBT: error
      creating funding TX: insufficient funds available to construct
      transaction

lntest/itest/lnd_psbt_test.go

@@ -1381,4 +1381,6 @@ func sendAllCoinsToAddrType(ctx context.Context, t *harnessTest,
 	require.NoError(t.t, err)
 
 	_ = mineBlocks(t, net, 1, 1)[0]
+	err = node.WaitForBlockchainSync()
+	require.NoError(t.t, err)
 }