Introduce unit tests

• Run unit tests in multisite mode as well
• Add badge to readme
• Abstract redirection functionality to make it testable

tests/test-admin-callbacks.php

+<?php
+/**
+ * Class AdminCallbacks
+ *
+ * @package External_Permalinks_Redux
+ */
+
+/**
+ * Test admin callbacks
+ */
+class AdminCallbacks extends WP_UnitTestCase {
+	/**
+	 * Redirect destination.
+	 */
+	const DESTINATION = 'https://w.org/';
+
+	/**
+	 * Redirect type.
+	 */
+	const TYPE = 302;
+
+	/**
+	 * Test post ID.
+	 *
+	 * @var int
+	 */
+	protected $post_id;
+
+	/**
+	 * Plugin instance.
+	 *
+	 * @var external_permalinks_redux
+	 */
+	protected $plugin;
+
+	/**
+	 * Metabox nonce.
+	 *
+	 * @var string
+	 */
+	protected $nonce;
+
+	/**
+	 * Create some objects with redirects.
+	 */
+	public function setUp() {
+		parent::setUp();
+
+		$this->plugin = external_permalinks_redux::get_instance();
+
+		$this->post_id = $this->factory->post->create(
+			[
+				'post_type' => 'post',
+			]
+		);
+
+		$this->nonce = wp_create_nonce( 'external-permalinks-redux' );
+	}
+
+	/**
+	 * Test metabox rendering.
+	 */
+	public function test_meta_box() {
+		ob_start();
+		$this->plugin->meta_box( get_post( $this->post_id ) );
+		$meta_box_contents = ob_get_clean();
+
+		$this->assertContains( 'value="' . $this->nonce . '"', $meta_box_contents );
+
+		foreach ( array_keys( $this->plugin->status_codes ) as $code ) {
+			$this->assertContains( 'value="' . $code . '"', $meta_box_contents );
+		}
+	}
+
+	/**
+	 * Test metabox save.
+	 */
+	public function test_save_callback() {
+		$_POST[ $this->plugin->meta_key_target . '_nonce' ] = $this->nonce;
+		$_POST[ $this->plugin->meta_key_target . '_url' ] = static::DESTINATION;
+		$_POST[ $this->plugin->meta_key_target . '_type' ] = static::TYPE;
+
+		$this->plugin->action_save_post( $this->post_id );
+
+		$this->assertEquals( static::DESTINATION, get_post_meta( $this->post_id, $this->plugin->meta_key_target, true ) );
+		$this->assertEquals( static::TYPE, get_post_meta( $this->post_id, $this->plugin->meta_key_type, true ) );
+	}
+}