It is already the case that native test code can be built with a single command (make test-image) and tests for multiple TEST.ROOTs can be run with a single jtreg invocation, but native test code for hotspot and jdk are in different directories, and there is no way to ask jtreg to look in both.

For openjdk, perhaps the very simple solution of merging images/test/{hotspot,jdk}/jtreg into a single directory images/test/jtreg is best. Hopefully there are no name collisions.