Adds a variant to a broadcast's split test, cloning source_broadcast_uuid's message content or, when omitted, the parent broadcast's own content. The clone always inherits the parent's audience and, when the source is a different broadcast, the parent's sending domain, from/reply-to addresses, EDS split, suppression lists and, when the parent has one, its footer. A split test takes at most 5 variants in total, counting the parent as variant 1. Rejected once the split test has launched, when the parent broadcast has no complete message, or when the account has no email footers.