[test][15-test_genec] Improve EC tests with genpkey

[test][15-test_genec] Improve EC tests with genpkey

Test separately EC parameters and EC key generation.

Some curves only support explicit params encoding.

For some curves we have had cases in which generating the parameters
under certain conditions failed, while generating and serializing a key
under the same conditions did not.
See <https://github.com/openssl/openssl/issues/12306> for more details.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12308)