Break some routines out of the mutable files test for future use

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Test for updating hosts files via links.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #8767 from erikh/update_parents_hosts_part2

Links test cleanup