RFR: 8237908: [TESTBUG] [macos] Add support to jtreg helpers to unpack pkg packages

[alexander . matveev]

Please review the jpackage fix for bug [1] at [2].

- Added support for unpacking pkg.

[1] https://bugs.openjdk.java.net/browse/JDK-8237908

[2] http://cr.openjdk.java.net/~almatvee/8237908/webrev.00/

Thanks,
Alexander