It may be hard to tell on others, especially when they threaten to hurt you, but you should tell the truth about what you saw in the forest. If you tell, then Abigail's lie won't get any bigger, and it won't put other people's lives in danger.
Lying about a sinful event isn't a good thing because that only creates more sins. Abigail pushed the blame away to protect herself and created a lie that would grow and affect the whole village. Abigail, Tituba, and Betty accused a bunch of people working with the Devil, which created chaos. If you find the courage to tell someone that they were dancing and conjuring, this lie would sizzle down and no one would be hanged for witchcraft. Being truthful is better than being sinful, and lie about what happened, isn't it?
However, Abigail did threaten to kill you if you told. I understand that that's scary, but do you really think she will be able to? Do you think she would be willing to pay the price when she gets caught? I don't think so because she's avoiding getting punished for dancing, and the punishment for dancing is being whipped. If she's scared of being whipped, I don't think she'll have the mind to murder anyone.
Have the courage to tell, and be truthful. You should stand up for what is right, even if you might be alone. You might save many people's lives.