Use order not degree to calculate a buffer size in ecdsatest

Otherwise this can result in an incorrect calculation of the maximum
encoded integer length, meaning an insufficient buffer size is allocated.

Thanks to Billy Brumley for helping to track this down.

Fixes #8209

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8237)