RFR 8153238: Improve test/tools/jlink/JLinkTest.java not to hard code the number of plugins

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8153238/webrev.00/ for
https://bugs.openjdk.java.net/browse/JDK-8153238

Thanks,

-Sundar