DPE: Test: Replace layer terminology with certificate

DPE spec does not have a notion of layers. Replace layer terminology
with certificate (or cert) in the code base.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: Icbdc0b9a4ae1bb20e7860eb85bcfd4b679a107b0
8 files changed