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>
diff --git a/test/config/default_s_test_config.cmake b/test/config/default_s_test_config.cmake
index 05ad57a..dfb103c 100644
--- a/test/config/default_s_test_config.cmake
+++ b/test/config/default_s_test_config.cmake
@@ -20,4 +20,5 @@
 
 set(TEST_S_FWU              OFF       CACHE BOOL      "Whether to build S regression FWU tests")
 set(TEST_S_IPC              OFF       CACHE BOOL      "Whether to build S regression IPC tests")
+set(TEST_S_SFN_BACKEND      OFF       CACHE BOOL      "Whether to build S regression SFN tests")
 set(TEST_S_FPU              OFF       CACHE BOOL      "Whether to build S regression FPU tests")