jtreg FAQ updates

[Jonathan Gibbons]

For those of you that run jtreg, I've made some updates to the jtreg FAQ
that is available in builds of jtreg, and on the OpenJDK website. [1]

In addition to some general cleanup, I've reorganized it a bit, to create a
couple of new sections, "3. Using jtreg" and "5. Organizing tests".
Most of section 3 is new, although a few questions were moved from 
elsewhere;
Other new questions include:
4.19 Can I (and should I) write shell tests?
5.5  How should I organize tests, libraries, and other test-related files?
5.6  Can I put more than one test in a file?

-- Jon

[1]  http://openjdk.java.net/jtreg/faq.html
(linked under "FAQ" from the main jtreg page 
http://openjdk.java.net/jtreg/)