ci: Update Corstone-1000 MPS3 sanity test

The Corstone-1000 MPS3 sanity test should ensure the board can boot to a
state where it is able to run tests.As the tests should be run as root
we need to ensure the default cassini user is able to switch to the root
user automatically when required.
The prompt string has also been updated to be consistent with the FVP
sanity script so that it no longer matches an earlier string in the boot
log.

Signed-off-by: Drew Reed <Drew.Reed@arm.com>