test/battery: adopt newly revamped setup()

test/plugins/battery.plugin.bats

@@ -1,9 +1,11 @@
-#!/usr/bin/env bats
+# shellcheck shell=bats
 
-load ../test_helper
-load ../test_helper_libs
+load "${MAIN_BASH_IT_DIR?}/test/test_helper.bash"
 
-load ../../plugins/available/battery.plugin
+function local_setup_file() {
+  setup_libs "helpers"
+  load "${BASH_IT?}/plugins/available/battery.plugin.bash"
+}
 
 # Sets up the `_command_exists` function so that it only responds `true` if called with
 # the name of the function that was passed in as an argument to `setup_command_exists`.