Add test coverage for normal alert parsing.

We have test coverage for invalid alerts, but not for normal ones on the DTLS
side.

Change-Id: I359dce8d4dc80dfa99b5d8bacd73f48a8e4ac310
Reviewed-on: https://boringssl-review.googlesource.com/3291
Reviewed-by: Adam Langley <agl@google.com>