RT#4625

(Imported from upstream's e3057a57caf4274ea1fb074518e4714059dfcabf.)

Add a test in ec_test to cover the ecp_nistz256_points_mul change. Also
revise the low-level infinity tests to cover the changes in
ecp_nistz256_point_add. Upstream's 'infty' logic was also cleaned up to
be simpler and take advantage of the only cases where |p| is infinity.

Change-Id: Ie22de834bf79ecb6191e824ad9fc1bd6f9681b8b
Reviewed-on: https://boringssl-review.googlesource.com/12225
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>