RFR: JDK-8179537 Update testing.md for more clarity regarding JTreg configuration

[Magnus Ihse Bursie]

Thanks to user feedback, this patch adds a section to testing.md about 
the need to configure the JTreg framework for run-test to be able to 
work, and describes some ways to achieve this.

Bug: https://bugs.openjdk.java.net/browse/JDK-8179537
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8179537-update-testing-readme-about-jtreg-configuration/webrev.01

Generated html file: 
http://cr.openjdk.java.net/~ihse/JDK-8179537-update-testing-readme-about-jtreg-configuration/webrev.01/raw_files/new/common/doc/testing.html

/Magnus