Rename DEV_MODE to GEN_FILES

GEN_FILES is a bit clearer as it describes what the setting
does more precisely.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 0bd94ba..41dceed 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -34,7 +34,7 @@
     list(APPEND generated_data_files ${CMAKE_CURRENT_BINARY_DIR}/${file})
 endforeach()
 
-if(DEV_MODE)
+if(GEN_FILES)
     add_custom_command(
         OUTPUT
             ${generated_data_files}