Test: Add Secure SFN Backend test

This patch adds Secure SFN Backend test (PSA API tests):
- Adding a new TEST_S_SFN_BACKEND_TEST config and enable by default
  for TEST_S
- Adding Secure test source and CMakeLists

Change-Id: Ife229c2c7c5abbf84effc18b0951f6a4eb5acace
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
11 files changed