RFR: JDK-8177955 Add testing documentation

Mon Apr 3 10:00:30 UTC 2017

The new run-test framework got a serious documentation effort alongside 
its creation. Unfortunately, there were at the time no good place to put 
that documentation.

Now that we have common/doc and a framework for handling build 
documentation, it would be good to put the run-test documentation there.

I intend to push this as a documentation bug (noreg-doc) into jdk9.

Bug: https://bugs.openjdk.java.net/browse/JDK-8177955
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8177955-add-testing-docs/webrev.01

/Magnus