The Test
Seth would never have considered letting an unworthy person test hid necklace. It was a very precious item for his tribe.
However, now that he knew it was a Relic of the Convergence and his tribe was probably created to protect this item on behalf of the Titans, he didn't see things the same way.
He still intended to keep the item with him until he found a member of his tribe who had the potential to replace it in the future. But he saw no problem with Ephraim's suggestion.
Seth's senses told him that there was no way a mere test could have negative consequences for him and his allies in terms of betrayal by Vlad and Ephraim.
Ephraim was too weak and would need a long time to master such an item, even if the conscience of the tribe's ancient elders in the item didn't work against him. He wouldn't be able to master such a thing right away, but he might be able to get a glimpse of what it was for.