|EC_POINT_mul| is almost exclusively used with reduced scalars, with
this exception. This comes from consumers following NIST SP 800-56A
section 5.6.2.3.2. (Though all our curves have cofactor one, so this
check isn't useful.)
Add a test for this so we don't accidentally break it.
Change-Id: I42492db38a1ea03acec4febdd7945c8a3933530a
Reviewed-on: https://boringssl-review.googlesource.com/23084
Reviewed-by: Adam Langley <agl@google.com>
0a5f006736