openarc.org provides infrastructure you may use to get familar with ARC.
Send a message to echo@openarc.org. You'll receive a response seconds later. This response contain the message as received by the service. This allowes you to inspect how OpenARC validate your sent message.
Send a message to forward@openarc.org. You'll receive your message forwarded back to you seconds later. It's not a new one. Existing DKIM signatures are expected to be still intact after forwarding.
But you may force message modification to invalidate your existing DKIM signature: Choose the text "modify" as Subject and it will be prepended with "[MODIFIED]: " on forwarding. Your DKIM signature will be no longer valid. We assume you sign the RFC5322.Subject header as advised by RFC 6376 - DKIM Signatures
You may also choose the text "add" as Subject and it will add a second Subject header into your message. That way you could see DKIM signatures stay valid unless you use oversigning. Background and DKIM SPEC
Subscribe to the mailing list first. Then you may send messages to openarc-testing@openarc.org. As usual your message is resend to all subscriber. There is no public list archive as that donsn't make sense.