[1.1.1][test] Avoid missing EC_GROUP wrappers

[1.1.1][test] Avoid missing EC_GROUP wrappers

Backport of https://github.com/openssl/openssl/pull/12096 to 1.1.1 broke
the build as the following functions are missing:

    const BIGNUM *EC_GROUP_get0_field(const EC_GROUP *group);
    int EC_GROUP_get_field_type(const EC_GROUP *group);

Turns out that for the purposes of the test code, we don't really need
to differentiate between prime and binary fields, and we can directly
use the existing `EC_GROUP_get_degree()` in the same fashion as was
being done for binary fields also for prime fields.

Fixes https://github.com/openssl/openssl/issues/12432

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12433)