Make Test_Widget compatible with WP_Widget

[adamziel]

Today I noticed that running certains e2e tests locally logs the following errors:

  ● Widgets Customizer › should handle the inserter outer section

    Unexpected error in page content: Warning: Declaration of Test_Widget::update($new_instance) should be compatible with WP_Widget::update($new_instance, $old_instance) in /var/www/html/wp-content/plugins/gutenberg-test-plugins/class-test-widget.php on line 65

      at visitAdminPage (../e2e-test-utils/build/@wordpress/e2e-test-utils/src/visit-admin-page.js:38:9)
          at runMicrotasks (<anonymous>)
      at Object.<anonymous> (specs/widgets/customizing-widgets.test.js:28:3

  ● Widgets Customizer › should move (inner) blocks to another sidebar

    Unexpected error in page content: Notice: Undefined index: title in /var/www/html/wp-content/plugins/gutenberg-test-plugins/class-test-widget.php on line 51

      at visitAdminPage (../e2e-test-utils/build/@wordpress/e2e-test-utils/src/visit-admin-page.js:38:9)
          at runMicrotasks (<anonymous>)
      at Object.<anonymous> (specs/widgets/customizing-widgets.test.js:28:3)

Turns out that the signature of Test_Widget::update is indeed incompatible with WP_Widget::update. This PR adjusts class Test_Widget to fix these two problems.

[gziolo]

Looks good and CI is green 👍🏻