test/system: Specify an explit return value

This removes any ambiguities and makes it clear what value is being
returned.

https://github.com/containers/toolbox/pull/1377

test/system/libs/helpers.bash

@@ -133,6 +133,9 @@ function _pull_and_cache_distro_image() {
   fi
 
   cleanup_all
+  ret_val="$?"
+
+  return "$ret_val"
 }