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>

include/openssl/poly1305.h

diff --git a/include/openssl/poly1305.h b/include/openssl/poly1305.h
index aa90486..32e109e 100644
--- a/include/openssl/poly1305.h
+++ b/include/openssl/poly1305.h
@@ -22,7 +22,7 @@
 #endif
 
 
-typedef unsigned char poly1305_state[512];
+typedef uint8_t poly1305_state[512];
 
 /* poly1305_init sets up |state| so that it can be used to calculate an
  * authentication tag with the one-time key |key|. Note that |key| is a