DEV: Introduce theme-qunit system spec (#23389)

This was intended to be included as part of c7dce90f43

spec/system/theme_qunit_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+describe "Theme qunit testing", type: :system do
+  let!(:theme_with_test) do
+    t = Fabricate(:theme, name: "My Theme")
+    t.set_field(target: :tests_js, type: :js, name: "acceptance/some-test.js", value: <<~JS)
+        import { module, test } from "qunit";
+        
+        module("theme test", function () {
+          test("it works", function (assert) {
+            assert.true(true)
+          });
+        });
+      JS
+    t.save!
+    t
+  end
+
+  it "can run theme tests correctly" do
+    visit "/theme-qunit"
+
+    find("a[href=\"/theme-qunit?id=#{theme_with_test.id}\"]").click
+
+    success_count = find("#qunit-testresult-display .passed").text
+    expect(success_count).to eq("1")
+  end
+end