FIX: Copy button JS test failures (#23687)

JS tests expect `show_copy_button_on_codeblocks` to be false (because
default before #81f3f56 was false). There is probably a different
issue at play here with JS tests, I haven't dug into it yet.

Instead, this PR adds a system test to ensure copy button is present
for code blocks with default site settings enabled.

app/assets/javascripts/discourse/tests/integration/components/widgets/post-cooked-test.js

@@ -8,6 +8,8 @@ module("Integration | Component | Widget | post-cooked", function (hooks) {
   setupRenderingTest(hooks);
 
   test("quotes with no username and no valid topic", async function (assert) {
+    this.siteSettings.show_copy_button_on_codeblocks = false;
+
     this.set("args", {
       cooked: `<aside class=\"quote no-group quote-post-not-found\" data-post=\"1\" data-topic=\"123456\">\n<blockquote>\n<p>abcd</p>\n</blockquote>\n</aside>\n<p>Testing the issue</p>`,
     });

spec/system/topic_page_spec.rb

@@ -33,4 +33,19 @@ describe "Topic page", type: :system do
       end
     end
   end
+
+  context "with a post containing a code block" do
+    before { Fabricate(:post, topic: topic, raw: <<~RAW) }
+      this a code block
+      ```
+      echo "hello world"
+      ```
+      RAW
+
+    it "includes the copy button" do
+      visit("/t/#{topic.slug}/#{topic.id}")
+
+      expect(".codeblock-button-wrapper").to be_present
+    end
+  end
 end