RFR: JDK-8180651: Make target to run tests on exploded image

[Erik Joelsson]

In most cases, when running regression tests, you want to run them on 
the jdk image because that's the correct image and some tests will not 
work on other variants. However, many tests can be run successfully on 
the exploded image and since building the full jdk image takes quite a 
bit of extra time, especially when running a slowdebug build, enabling 
this as an option can significantly improve developer efficiency.

I therefor propose the new target "exploded-run-test". It will work just 
like run-test but it depends on and runs on the exploded image.

Bug: https://bugs.openjdk.java.net/browse/JDK-8180651

Webrev: http://cr.openjdk.java.net/~erikj/8180651/webrev.01/

/Erik