Support Wycheproof vectors with the curve given in the group.
Future versions of the Wycheproof vectors will specify the curve for a
group of tests, rather than for each test. This change works with both
the old and new style.
Change-Id: I0d9a503c8357eb4c617544e727d8f4a703c2c2b0
Reviewed-on: https://boringssl-review.googlesource.com/30084
Reviewed-by: Adam Langley <agl@google.com>