Fix build errors in CMake

tests/src/ssl_helpers.c depends on functions defined
in library/*.c. If it's complied as an OBJECT with other c files,
cmake complains undefined reference in link stage under programs/.
Therefore, tests/src/test_helpers/ is created to hold c files with
dependency of library/*.c. Besides, tests/src/test_helper/*.c is
separated into another OBJECT, mbedtls_test_helpers, as sources
to build all test suite executables.

In addition, everest header directory is included in case
MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED is enabled.

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 519604b..7bd9189 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -280,12 +280,23 @@
 # to define the test executables.
 #
 if(ENABLE_TESTING OR ENABLE_PROGRAMS)
-    file(GLOB MBEDTLS_TEST_FILES ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/*.c ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/drivers/*.c)
+    file(GLOB MBEDTLS_TEST_FILES
+         ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/*.c
+         ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/drivers/*.c)
     add_library(mbedtls_test OBJECT ${MBEDTLS_TEST_FILES})
     target_include_directories(mbedtls_test
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/tests/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include
         PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/library)
+
+    file(GLOB MBEDTLS_TEST_HELPER_FILES
+         ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/test_helpers/*.c)
+    add_library(mbedtls_test_helpers OBJECT ${MBEDTLS_TEST_HELPER_FILES})
+    target_include_directories(mbedtls_test_helpers
+        PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/tests/include
+        PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include
+        PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/library
+        PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/3rdparty/everest/include)
 endif()
 
 if(ENABLE_PROGRAMS)