Add PHPUnit testing framework and error checking documentation

tests/phpunit/bootstrap.php

@@ -0,0 +1,22 @@
+<?php
+/**
+ * PHPUnit bootstrap file.
+ *
+ * @package WP_Plugin_Starter_Template_For_AI_Coding
+ */
+
+// Composer autoloader must be loaded before WP_PHPUNIT__DIR will be available.
+require_once dirname( dirname( __DIR__ ) ) . '/vendor/autoload.php';
+
+// Give access to tests_add_filter() function.
+require_once getenv( 'WP_PHPUNIT__DIR' ) . '/includes/functions.php';
+
+/**
+ * Manually load the plugin being tested.
+ */
+function _manually_load_plugin() {
+	require dirname( dirname( __DIR__ ) ) . '/plugin-toggle.php';
+}
+
+// Start up the WP testing environment.
+require getenv( 'WP_PHPUNIT__DIR' ) . '/includes/bootstrap.php';