DEV: De-arrowify tests (#11068)

Using arrow functions changes `this` context, which is undesired in tests, e.g. it makes it impossible to setup things like pretender (`this.server`) in `beforeEach` hooks.

Ember guides always use classic functions in examples (e.g. https://guides.emberjs.com/release/testing/test-types/), and that's what it uses in its own test suite, as do various addons and ember apps.

It was also already used in Discourse where `this` was required. Moving forward, it will be needed in more places as we migrate toward ember-cli.

(I might later add a custom rule to eslint-discourse-ember to enforce this)

plugins/poll/test/javascripts/acceptance/poll-pie-chart-test.js.es6

@@ -8,7 +8,7 @@ acceptance("Rendering polls with pie charts", function (needs) {
     poll_groupable_user_fields: "something",
   });
 
-  test("Displays the pie chart", async (assert) => {
+  test("Displays the pie chart", async function (assert) {
     await visit("/t/-/topic_with_pie_chart_poll");
 
     const poll = queryAll(".poll")[0];