oeqa: Add Linux Shutdown OEQA test

Previously, Kronos software stack did not differentiate between
reset and shutdown requests. A shutdown request resulted in
a system reset which was an incorrect behavior.

Now, a shutdown request using `shutdown now` in the Linux
console can successfully shutdown the system and eventually
terminate the FVP.

Add an OEQA test to verify this behavior which runs at the
end of automated validation in all use cases.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>