Update package name and add unit test for WritePoetry class

tests/test-sample.php

@@ -2,7 +2,7 @@
 /**
  * Class SampleTest
  *
- * @package Write_Poetry
+ * @package WritePoetry
  */
 
 /**

tests/test-writepoetry.php

@@ -0,0 +1,36 @@
+<?php
+/**
+ * Class SampleTest
+ *
+ * @package WritePoetry
+ */
+
+/**
+ * Sample test case.
+ */
+class WritePoetryTest extends WP_UnitTestCase {
+
+	/**
+	 * A single example test.
+	 */
+	public function set_up() {
+        parent::set_up();
+
+        // Mock that we're in WP Admin context.
+		// See https://wordpress.stackexchange.com/questions/207358/unit-testing-in-the-wordpress-backend-is-admin-is-true
+        set_current_screen( 'edit-post' );
+
+        $this->write_poetry = WritePoetry\Init::register_services();
+
+    }
+
+	public function tear_down() {
+        parent::tear_down();
+    }
+
+	public function test_has_settings_interface() {
+		$has_settings_interface = ( is_a( $this->write_poetry->settings, 'Starter_Plugin_Settings' ) );
+
+		$this->assertTrue( $has_settings_interface );
+	}
+}