The coverage tool revealed that we weren't testing all codepaths of the ChaCha
assembly. Add a standalone test as it's much easier to iterate over all lengths
when there isn't the entire AEAD in the way.
I wasn't able to find a really long test vector, so I generated a random one
with the Go implementation we have in runner.
This test gives us full coverage on the ChaCha20_ssse3 variant. (We'll see how
it fares on the other codepaths when the multi-variant test harnesses get in. I
certainly hope there isn't a more novel way to call ChaCha20 than this...)
Change-Id: I087e421c7351f46ea65dacdc7127e4fbf5f4c0aa
Reviewed-on: https://boringssl-review.googlesource.com/7299
Reviewed-by: Adam Langley <agl@google.com>
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