RFR: JDK-8198323 testing.md not updated for repository layout change

[Magnus Ihse Bursie]

In make/doc/testing.md, the descriptions of the TEST option for jtreg 
tests has not been updated to account for the repository layout change.  
For example, "hotspot/test:hotspot_gc" should be 
"test/hotspot/jtreg:hotspot_gc".

Bug: https://bugs.openjdk.java.net/browse/JDK-8198323
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8198323-update-testing-documentation/webrev.01

/Magnus