DEV: Fix invalid test in 5dba5c4208ee8560090c815729b3053b9fd82585 (#25810)

Why this change?

The previous test setup did not actually act as a regression test
because the test will still pass even when the fix is removed. This
commit uses a rendering test instead to ensure that Ember is tracking
the property change.

app/assets/javascripts/discourse/tests/unit/models/extra-nav-item-test.gjs

@@ -0,0 +1,24 @@
+import { render, settled } from "@ember/test-helpers";
+import { setupRenderingTest } from "ember-qunit";
+import { module, test } from "qunit";
+import { ExtraNavItem } from "discourse/models/nav-item";
+
+module("Unit | Model | extra-nav-item", function (hooks) {
+  setupRenderingTest(hooks);
+
+  test("displayName updates when count property changes", async function (assert) {
+    const extraNavItem = ExtraNavItem.create({
+      name: "something",
+    });
+
+    await render(<template><p>{{extraNavItem.displayName}}</p></template>);
+
+    assert.dom("p").hasText("[en.filters.something.title count=0]");
+
+    extraNavItem.count = 2;
+
+    await settled();
+
+    assert.dom("p").hasText("[en.filters.something.title_with_count count=2]");
+  });
+});

app/assets/javascripts/discourse/tests/unit/models/extra-nav-item-test.js

@@ -1,25 +0,0 @@
-import { setupTest } from "ember-qunit";
-import { module, test } from "qunit";
-import { ExtraNavItem } from "discourse/models/nav-item";
-
-module("Unit | Model | extra-nav-item", function (hooks) {
-  setupTest(hooks);
-
-  test("displayName updates when count property changes", function (assert) {
-    const extraNavItem = ExtraNavItem.create({
-      name: "something",
-    });
-
-    assert.strictEqual(
-      extraNavItem.displayName,
-      "[en.filters.something.title count=0]"
-    );
-
-    extraNavItem.count = 2;
-
-    assert.strictEqual(
-      extraNavItem.displayName,
-      "[en.filters.something.title_with_count count=2]"
-    );
-  });
-});