See the long discussion beginning here:
http://mail.openjdk.java.net/pipermail/jtreg-use/2016-November/000510.html

And this specific message here:
http://mail.openjdk.java.net/pipermail/jtreg-use/2016-November/000523.html

TL;DR: from Christoph Langer:
I agree that it would be nicer if jtreg would leave the jtreg lib path as java property to be able to elevate all of its contents. But the current proposal with a set of TEST_JARS of jtreg, javatest and testng is probably sufficient for jaxp testing.