The Friar says this when he gives Juliet the potion. It means that she'll seem dead—her body will be cold, she won't breath, and her lips and cheeks will be very pale.
In context, there are several literary devices used:
Personification (warmth doesn't "testify")
Perhaps hyperbole (there will likely still be SOME warmth in the body)
Imagery (wanny ashes)
Metaphor (there are no literal roses in her cheeks)