RFR: JDK-8164301: jib should provide a JDK to for running jtreg with

[Erik Joelsson]

When running tests via "jib make --profile run-test", jib does not set 
JT_JAVA (the JDK used to run the jtreg harness). This change makes Jib 
set the bootjdk as JT_JAVA. It also makes the same change in the top 
level makefiles when running "make test".

Bug: https://bugs.openjdk.java.net/browse/JDK-8164301

Webrev: http://cr.openjdk.java.net/~erikj/8164301/webrev.01/

/Erik