Add test for duplicate HTLC fail-back

[TheBlueMatt]

In 0.1.1 we fixed an issue where if someone relays two identical (hash and amount) HTLCs through us and then force-closes with a stale state we'll only fail-back one of them. We fixed it indirectly by just failing back everything at a timeout, which is fine, but we shouldn't wait to fail these HTLCs back. Further, we should add a test for this.