Change variable name to lowercase

This seems more correct for variables that are only used in
a small area of the cmake file.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index ef41f29..ef484f6 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -25,10 +25,10 @@
         WORKING_DIRECTORY
             ${CMAKE_CURRENT_SOURCE_DIR}/..
         OUTPUT_VARIABLE
-            TEST_SUITE_DATA_FILES)
+            test_suite_data_files)
     add_custom_command(
         OUTPUT
-            ${TEST_SUITE_DATA_FILES}
+            ${test_suite_data_files}
         WORKING_DIRECTORY
             ${CMAKE_CURRENT_SOURCE_DIR}/..
         COMMAND