cmake: Move cert.o to mbedtls_test_helpers library of objects

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 70706a6..68a3912 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -413,6 +413,7 @@
 #
 if(ENABLE_TESTING OR ENABLE_PROGRAMS)
     file(GLOB MBEDTLS_TEST_HELPER_FILES
+         ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/*.c
          ${CMAKE_CURRENT_SOURCE_DIR}/tests/src/test_helpers/*.c)
     add_library(mbedtls_test_helpers OBJECT ${MBEDTLS_TEST_HELPER_FILES})
     set_base_compile_options(mbedtls_test_helpers)