it will be free for everyone as the purpose is to make people test it so...
Restoration is a perfectly valid school of magic, and don't let anyone tell you otherwise! - Spells and incantations for those with the talent to cast them!
I'm pretty sure I heard, and it was really only a quick mumble about it while they were talking about the PTS launch on the twitch stream, that subscribers will get access to PTS just like normal.. so.. ESO+ members will have access to PTS, others that are going standard B2P won't have access. This is all speculation though so.. yeah.
they said on the announcement live stream that after march 17th it'll be open to all who have purchased a copy of the game but up until then, only active subscribers will have access to it.
"It is a time of strife and unrest. Armies of revenants and dark spirits manifest in every corner of Tamriel. Winters grow colder and crops fail. Mystics are plagued by nightmares and portents of doom."