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/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ca477ac..10de8c7 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -50,7 +50,9 @@
         DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/scripts/generate_test_code.py ${mbedtls_target} ${CMAKE_CURRENT_SOURCE_DIR}/suites/helpers.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/main_test.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/host_test.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${suite_name}.function ${CMAKE_CURRENT_SOURCE_DIR}/suites/test_suite_${data_name}.data
     )
 
-    add_executable(test_suite_${data_name} test_suite_${data_name}.c $<TARGET_OBJECTS:mbedtls_test>)
+    add_executable(test_suite_${data_name} test_suite_${data_name}.c
+                   $<TARGET_OBJECTS:mbedtls_test>
+                   $<TARGET_OBJECTS:mbedtls_test_helpers>)
     target_link_libraries(test_suite_${data_name} ${libs})
     # Include test-specific header files from ./include and private header
     # files (used by some invasive tests) from ../library. Public header