Fix CMake change failures on Windows

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index edb513c..dcc5de0 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -40,8 +40,9 @@
         psa_generated_data_files)
 
 # Derive generated file paths in the build directory
+set(base_generated_data_files ${bignum_generated_data_files} ${psa_generated_data_files})
 set(generated_data_files "")
-foreach(file ${bignum_generated_data_files} ${psa_generated_data_files})
+foreach(file ${base_generated_data_files})
     list(APPEND generated_data_files ${CMAKE_CURRENT_BINARY_DIR}/${file})
 endforeach()