test/system: Limit the scope of the return value to the function

This should prevent this function from overwriting variables of the
same name beyond the function and causing hard-to-debug problems.

https://github.com/containers/toolbox/pull/1372

test/system/libs/helpers.bash

@@ -439,7 +439,7 @@ function container_started() {
   start_container "$container_name"
 
   # Used as a return value
-  container_initialized=1
+  local -i container_initialized=1
 
   local -i j
   local num_of_retries=5