cassini-distro: Enable `root` account for `cassini-test`

In Cassini distribution, when `cassini-security` distro feature is
enabled, `root` account is disabled.

Current LAVA test framework (inline test action) needs to run
`transfer_overlay` action if there are any shell commands in the inline
test action. In order for `transfer_overlay` to work `root` account
is required because, overlay is done at `/`. Therefore, do not
disable `root` account when `cassini-security` and `cassini-test` distro
features are enabled.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>