Make it clear this is not a pristine full copy of all of Wycheproof as a
library.

Change-Id: I1aa5253a1d7c696e69b2e8d7897924f15303d9ac
Reviewed-on: https://boringssl-review.googlesource.com/28188
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>