Add a test for underflow in ecp_nistp521.c

The previous commit fixed an underflow that may occur in ecp_nistp521.c.
This commit adds a test for that condition. It is heavily based on an
original test harness by Billy Brumley.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/8405)