[test] unit test for field_inv function pointer in EC_METHOD

This is a rewrite of commit 8f58ede0

 for
the 1.1.0-stable branch.

Co-authored-by: Billy Brumley <bbrumley@gmail.com>

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8263)