test/commit: Simplify a 'for' loop

An ascending 'for' loop is more idiomatic than its descending
counterpart.

https://github.com/containers/toolbox/pull/1373

test/system/libs/helpers.bash

@@ -107,7 +107,7 @@ function _pull_and_cache_distro_image() {
     assert_success
   fi
 
-  for ((i = num_of_retries; i > 0; i--)); do
+  for ((i = 0; i < num_of_retries; i++)); do
     run "$SKOPEO" copy --dest-compress "docker://${image}" "dir:${IMAGE_CACHE_DIR}/${image_archive}"
 
     if [ "$status" -eq 0 ]; then