ci: add df/skip-rootfs test to SMACK/ROOTFS CI

[ChrisDryden]

So from first glance this PR seems more complicated than it actually is, the only functional change in the whole PR is adding

--package uu_df

and

Adding rootfs in mtab to the grep to search for tests. The rest of the changes are renaming all of the places I named the CI runner the "SMACK" runner to reflect that the tests being run here are the QEMU tests that will use QEMU to emulate environments for the more unique environments.

This doesn't actually make any fixes for the rootfs test, it just lets us run it on the SMACK host that happens to have rootfs visible.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/df/skip-rootfs was skipped on 'main' but is now failing.