Use PKCS#8 format EC key so test is skipped with no-ec

Reviewed-by: NRich Salz <rsalz@openssl.org>

test/evptests.txt

@@ -2257,11 +2257,11 @@ x/iUJAcsJxl9eLM7kg6VzbZk6ZDc8M/qDZTiqOavnQ5YBW5lMQSSW5/myQ==
 
 # Additional EC key for ECDH
 PrivateKey=P-256-Peer
------BEGIN EC PRIVATE KEY-----
-MHcCAQEEIP1Ec7tUwzcFBVmd4nSyEgGaRGNL3ydqSwe3/l548pdjoAoGCCqGSM49
-AwEHoUQDQgAEIAbl4UoHiqc/uKYl5WElS78tDU/sCrmVlCNy6lFAldO8IPkSyzGP
-MjSvZI6nIGQ7P3Afm3reuJ0KvnTZ77acRg==
------END EC PRIVATE KEY-----
+-----BEGIN PRIVATE KEY-----
+MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQg/URzu1TDNwUFWZ3i
+dLISAZpEY0vfJ2pLB7f+Xnjyl2OhRANCAAQgBuXhSgeKpz+4piXlYSVLvy0NT+wK
+uZWUI3LqUUCV07wg+RLLMY8yNK9kjqcgZDs/cB+bet64nQq+dNnvtpxG
+-----END PRIVATE KEY-----
 
 PublicKey=P-256-Peer-PUBLIC
 -----BEGIN PUBLIC KEY-----