DPE: Update test to check for return error type

If a required argument is omitted, then check if
DPE_INVALID_ARGUMENT is returned.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I690273e5ef4715ddcbf0cc4e17e1f96794dde2eb
1 file changed