Feature #8415: Migrate from aufs to overlayfs
Create overlayfs-based IUKs v2 for the test suite
See 5f13cb87adb0d80c5dca4d364eb8852439789dfd for details.
Test suite: adjust create-test-iuks for overlayfs version numbers (refs: #17440)
For a while we'll have 2 kinds of:
- IUKs v2 on the download mirrors
- UDFs v2 on our website
i.e. one set based on aufs, the other set based on overlayfs.
They are differentiated by having their version number end with ~test vs.
~testoverlayfs, which yields different filenames.
Test suite: sync create-test-iuks script to include the expected changes (refs: #17440)