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/discourse-local-dates/test/javascripts/acceptance/local-dates-composer-test.js.es6

@@ -4,7 +4,7 @@ acceptance("Local Dates - composer", function (needs) {
   needs.user();
   needs.settings({ discourse_local_dates_enabled: true });
 
-  test("composer bbcode", async (assert) => {
+  test("composer bbcode", async function (assert) {
     const getAttr = (attr) => {
       return queryAll(
         ".d-editor-preview .discourse-local-date.cooked-date"