Force WordPress to check for plugin and theme updates on every page load. Obviously used for testing. Do not use this full time as it will slow your site down.

update-transient-flusher.php

<?php
/*
Plugin Name: Updates Transient Flusher
Description: Deletes the update_themes and update_plugins transients. Forces WordPress to check for plugin and theme updates on every page load. FOR TESTING ONLY.
*/

function jp_delete_update_transients() {

	delete_transient( 'update_themes' );
	delete_transient( 'update_plugins' );
	delete_site_transient( 'update_plugins' );
	delete_site_transient( 'update_themes' );
}
add_action( 'init', 'jp_delete_update_transients' );

With WP CLI:

wp transient delete update_themes
wp transient delete update_plugins
wp transient delete update_themes --network
wp transient delete update_plugins --network