cassini-tests: Implement cassini tests

These are the following integration tests added:
* container-engine
* k3s
* user-accounts

Issue-Id: OSSEMBEDA-1631
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Richard Neill <richard.neill@arm.com>
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Signed-off-by: Vikas Katariya <vikas.katariya@arm.com>
Change-Id: I23342589b1214a58143a62689efb373fba1ad014