[10]RFR: 8136650: Add support for custom jtreg native tests

[Robbin Ehn]

Hi, please review.

This adds support for custom extensions the jtreg native tests.

The issues is closed due to internal tests but contains no more info than this subject:
https://bugs.openjdk.java.net/browse/JDK-8136650

Open webrev:
http://cr.openjdk.java.net/~rehn/8136650/

Tested with dummy native test.

Thanks, Robbin