Move convert_wycheproof.go to util/

This file is not part of the Wycheproof project and consumers of
BoringSSL who wish to provide Wycheproof themselves (and not have
third_party/wycheproof_testvectors) need it in another location.

Change-Id: I730fe294f46a9aac77b858a91a03ee64fb8ea579
Reviewed-on: https://boringssl-review.googlesource.com/28704
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>