Review Request JDK-8169909 java agent fails to add to class path when the initial module is a named module

[Mandy Chung]

http://cr.openjdk.java.net/~mchung/jdk9/webrevs/8169909/webrev.00/

This fixes a regression that causes java agent fails to 
append to class path when the initial module on the module
path but no class path.  The AppClassLoader should have a 
URLClassPath instance set for the support of Java agent,
Instrumentation::appendToSystemClassLoaderSearch.

This patch also updates JavaClassPathTest to cover more test
cases for JAR file and CLASS-PATH attribute.

Mandy
[1] http://mail.openjdk.java.net/pipermail/jdk9-dev/2016-November/005213.html