Add standalone tests for Poly1305.

RFC 7359 includes tests for various edge cases. Also, as
CRYPTO_poly1305_update can be used single-shot and streaming, we should
explicitly stress both.

Change-Id: Ie44c203a77624be10397ad05f06ca98d937db76f
Reviewed-on: https://boringssl-review.googlesource.com/5410
Reviewed-by: Adam Langley <agl@google.com>