"Testimony needs corroboration by independent witnesses,"

Yes, in law this is enough to put someone to death. By comparison within certain spiritual traditions evidence described as 'verifiable personal gnosis' is contrasted to 'unverifiable personal gnosis'. With the former, personal experience is tested against and corroborated by the testimony of others. With the latter, personal experience is tested against and not corroborated by the testimony of others. Clearly the former is considered more valid as evidence of genuine gnosis or direct spiritual experience, than the latter.