DEV: Fix some deprecation warnings in tests

- Import `exists()` instead of using the global function (#13010)

app/assets/javascripts/discourse/tests/integration/components/hidden-details-test.js

@@ -1,7 +1,11 @@
 import componentTest, {
   setupRenderingTest,
 } from "discourse/tests/helpers/component-test";
-import { discourseModule, query } from "discourse/tests/helpers/qunit-helpers";
+import {
+  discourseModule,
+  exists,
+  query,
+} from "discourse/tests/helpers/qunit-helpers";
 import hbs from "htmlbars-inline-precompile";
 import I18n from "I18n";
 

app/assets/javascripts/discourse/tests/integration/components/slow-mode-info-test.js

@@ -3,6 +3,7 @@ import componentTest, {
 } from "discourse/tests/helpers/component-test";
 import {
   discourseModule,
+  exists,
   queryAll,
 } from "discourse/tests/helpers/qunit-helpers";