There are junit jpackage tests for shared code. Need to facilitate junit tests for platform-specific jpackage classes, i.e., classes that exist only on one platform.

Move the shared junit tests from "test/jdk/tools/jpackage/junit" folder into "test/jdk/tools/jpackage/junit/share" folder. Platform-specific tests will be added in "test/jdk/tools/jpackage/junit/$platform" folders.