Collect changes to jpackage from different work-in-progress fixes that don't affect its behavior:

    Use --release 11 javac option to compile test app. This allows custom runtimes created from older JDKs to be used. Using external runtimes allows to avoid jpackage running jlink in every test and saves time in local tests. With this change, creating a single stub runtime and using it with jpackage tests across multiple releases is possible.
    Add "JUnitUtils" helper.
    Add "ObjectMapper" class that converts a given object into a property map (Map<String, Object>). Handy to create a dump of object hierarchies.
    Add "ConfigurationTarget" class to simplify applying the same initializers when testing app image and native package bundling.
    Fix "AppImagePackageTest" test to make it produce a valid macOS bundle to leverage the default verification of jpackage output in the jpackage test lib.
    Add test cases for the main/additional launcher descriptions to AddLauncherTest test.
    Add validation of the "Comment" key value in Linux .desktop files.