https://boringssl-review.googlesource.com/6101 was mismerged from *ring* and
lost some tests. Also add the corresponding tag truncation tests for the new
construction. So long as we have that feature, we should have tests for it.
(Although, do we actually need to support it?)
Change-Id: I70784cbac345e0ad11b496102856c53932b7362e
Reviewed-on: https://boringssl-review.googlesource.com/6682
Reviewed-by: Adam Langley <agl@google.com>
