TKit.createTempFile() [1] allows the creation of a unique file in the root test work directory only, and TKit.createTempDirectory() [2] can create a directory in the subdirectory inside of the test work directory.

It would be better to have these functions unified. Also, remove `throws IOEXception` from their signatures - wrap java.io.IOException-s in java.io.UncheckedIOException-s.

Make TKit.assertDirectoryExists(Path, Optional<Boolean>) [3] private. It is internal and shouldn't be exposed.

Add a check for the main jar in the app image if the main jar was specified to JPackageCommand class. This is a missing part of JDK-8325089.

[1] https://github.com/openjdk/jdk/blob/65f79c145b7b1b32ed064a37ad4d2b6aca935a4c/test/jdk/tools/jpackage/helpers/jdk/jpackage/test/TKit.java#L330
[2] https://github.com/openjdk/jdk/blob/65f79c145b7b1b32ed064a37ad4d2b6aca935a4c/test/jdk/tools/jpackage/helpers/jdk/jpackage/test/TKit.java#L323
[3] https://github.com/openjdk/jdk/blob/65f79c145b7b1b32ed064a37ad4d2b6aca935a4c/test/jdk/tools/jpackage/helpers/jdk/jpackage/test/TKit.java#L706