RFR: JDK-8193061 Add run-test-prebuilt functionality

[Magnus Ihse Bursie]

This addition makes it possible to run tests using the normal 
RunTests.gmk framework, even if configure has not been run. The typical 
use case is for automated testing, where a test machine can download a 
prebuilt JDK image and test image, and the source tree, and then start 
executing tests as specified by RunTests.gmk in the source tree, but 
excersicing the prebuilt JDK and test image.

This mode of operation has historically been provided by test/Makefile, 
but that system is messy, not maintained and parallel to the run-test 
implementation.

With this patch, a new "global" target "run-test-prebuilt" is provided. 
To use it, you must also specify a number of variables, either on the 
command line or in the environment. These are the variables that 
run-test-prebuilt cannot guess. With these values as base, 
run-test-prebuilt creates a "fake" spec file, with some generic values 
that are likely to work in many cases, and/or values that can be 
detected in runtime.

Bug: https://bugs.openjdk.java.net/browse/JDK-8193061
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8193061-add-run-test-prebuilt/webrev.01

/Magnus