Use SSL_get_cipher_by_value in cipher_get_rfc_name tests.

Saves making a temporary SSL_CTX and looking at its insides.

Change-Id: Ia351b9b91aec8b813ad7b6e373773396f0975f9a
Reviewed-on: https://boringssl-review.googlesource.com/3561
Reviewed-by: Adam Langley <agl@google.com>