tests: adapt common_dom to files_provider
Also start to consider the "files" provider when cleaning up the files.
This change will let us start to moving to "files" provider in our test
suite.
Related:
https://pagure.io/SSSD/sssd/issue/3304
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
Reviewed-by: Pavel Březina <pbrezina@redhat.com>