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.