RFR: JDK-8233270: Add support to jtreg helpers to unpack packages

[Alexey Semenyuk]

Please review  fixes for [1] and [2]. Both target jpackage tool.

The webrev is at [3].

[1] https://bugs.openjdk.java.net/browse/JDK-8233270

[2] https://bugs.openjdk.java.net/browse/JDK-8230933

[3] http://cr.openjdk.java.net/~asemenyuk/8233270/webrev.00/