Build: Correct usage of out-of-tree test suite build

Extra test suite build currently doesn't support to build
multiple test suites from different directories. The cmake
configuration names and build system are updated.

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: I6f8b28efc3c6c5d1187586264a4cb76deae455de
diff --git a/test/secure_fw/secure_tests.cmake b/test/secure_fw/secure_tests.cmake
index c74ec5a..c5f6dd5 100644
--- a/test/secure_fw/secure_tests.cmake
+++ b/test/secure_fw/secure_tests.cmake
@@ -94,6 +94,6 @@
 if (TEST_S_FPU)
     add_library(tfm_test_suite_fpu_s STATIC EXCLUDE_FROM_ALL)
 endif()
-if (EXTRA_S_TEST_SUITES_PATHS)
+if (EXTRA_S_TEST_SUITE_PATH)
     add_library(tfm_test_suite_extra_s STATIC EXCLUDE_FROM_ALL)
 endif()