tests: update for rename of CentOS8 PowerTools repo
This was intentionally renamed recently to be all lowercase:
https://bugs.centos.org/view.php?id=17920
https://wiki.centos.org/Manuals/ReleaseNotes/CentOS8.2011#Yum_repo_file_and_repoid_changes
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <
20201216141653.213980-1-berrange@redhat.com>
[AJB: bump up FROM to trigger re-build, add diffutils]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>