[foreign-jextract] RFR 8238466: add jtreg test driver for jextract tool

[sundararajan.athijegannathan at oracle.com]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8238466

Webrev: http://cr.openjdk.java.net/~sundar/8238466/webrev.00/

PS. Piggybacking to fix a minor issue with vararg handling

Thanks,

-Sundar