RFR: JDK-8235252: Repair run_tests.sh by locating jtreg locally

[Alexander Matveev]

Please review simple script fix [2] for jpackage bug [1].
 - Modified script to download jtreg bundle from URL provided by 
environment variable JTREG_BUNDLE_URL.

Thanks,
Alexander

[1] https://bugs.openjdk.java.net/browse/JDK-8235252

[2] http://cr.openjdk.java.net/~almatvee/8235252/webrev.00/