Don't try restoring a file if no backup is available

This caused `all.sh --force` to fail on a clean build tree.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/scripts/all.sh

@@ -303,7 +303,9 @@ cleanup()
 
     # Restore files that may have been clobbered by the job
     for x in $files_to_back_up; do
-        cp -p "$x$backup_suffix" "$x"
+        if [[ -e "$x$backup_suffix" ]]; then
+            cp -p "$x$backup_suffix" "$x"
+        fi
     done
 }