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-details/test/javascripts/lib/details-cooked-test.js.es6

@@ -13,7 +13,7 @@ const defaultOpts = buildOptions({
   getURL: (url) => url,
 });
 
-test("details", (assert) => {
+test("details", function (assert) {
   const cooked = (input, expected, text) => {
     assert.equal(
       new PrettyText(defaultOpts).cook(input),