Fix out-of-source build fail

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 41dceed..8bb6250 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -255,4 +255,5 @@
     link_to_source(data_files)
     link_to_source(scripts)
     link_to_source(ssl-opt.sh)
+    link_to_source(opt-testcases)
 endif()