DPE: Fix all test failures

- For reg test, increase number of layers to 6.
- Separate certificate IDs for all the tests where undestroyable
  contexts are created.
- For now, until allocated buffer sizes are passed to the service, return valid
  context handle if DPE service has successfully derived the context.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I5766b0df21d79dff569789aedee94bd50c2b9d9a
6 files changed