Batch callback function for testing the theme used on the progress page.
function _batch_test_theme_callback() {
// Because drupalGet() steps through the full progressive batch before
// returning control to the test function, we cannot test that the correct
// theme is being used on the batch processing page by viewing that page
// directly. Instead, we save the theme being used in a variable here, so
// that it can be loaded and inspected in the thread running the test.
global $theme;
batch_test_stack($theme);
}