public function test_register() {
    register_setting( 'test_group', 'test_option' );

    $registered = get_registered_settings();
    $this->assertArrayHasKey( 'test_option', $registered );

    $args = $registered['test_option'];
    $this->assertSame( 'test_group', $args['group'] );

    // Check defaults.
    $this->assertSame( 'string', $args['type'] );
    $this->assertFalse( $args['show_in_rest'] );
    $this->assertSame( '', $args['description'] );
  }