Test various cases where plaintexts and ciphertexts are too large.

Note that DTLS treats oversized ciphertexts different from everything else.

Change-Id: I71cba69ebce0debdfc96a7fdeb2666252e8d28ed
Reviewed-on: https://boringssl-review.googlesource.com/5786
Reviewed-by: Adam Langley <agl@google.com>