RFR: 8147607: Remove test library dependency on sun.security.tools.jarsigner.Main

[Steve Drach]

Please review a small change to the zipfs test library.

Issue: https://bugs.openjdk.java.net/browse/JDK-8147607 <https://bugs.openjdk.java.net/browse/JDK-8147607>
Webrev: http://cr.openjdk.java.net/~sdrach/8147607/webrev <http://cr.openjdk.java.net/~sdrach/8147607/webrev>

This change uses the public jdk.security.jarsigner.JarSigner API rather than the API exposed by sun.security.tools.jarsigner.