Add UnauthenticatedECDH bug test.

This works, but there's enough shared codepaths that it's worth a test to
ensure it stays that way.

Change-Id: I5d5a729811e35832170322957258304213204e3b
Reviewed-on: https://boringssl-review.googlesource.com/1155
Reviewed-by: Adam Langley <agl@google.com>