[9] RFR 8068290: JCK: 2 api/java_security/cert/ tests fail

[Vincent Ryan]

Please review this change to correct two failing JCK tests.

Bug: https://bugs.openjdk.java.net/browse/JDK-8068290
Webrev: http://cr.openjdk.java.net/~vinnie/8068290/webrev.00/

PKCS12 is the default keystore type in JDK 9 and its implementation stores only X.509 certificates
although the KeyStore API allows any Certificate object to be stored.
This fix checks that the supplied certificate is an X509Certificate object before storing it.

Thanks.