It would help to add a command line option to jtreg to provide the ability to clean the work directory before a test run, and the classes subdirectory in particular.

Suggested:

--clean-work-directory VALUE

VALUE is one of

none -- the current behavior
classes -- the classes subdirectory, and proposed new default
all -- remove all pre-existing files from the work directory, except maybe the `jtData` directory, depending on impl difficulty