Use an existing image for testing /help

In spec/controllers/help_controller_spec.rb we test how an image is
rendered under /help. Given that we usually rename images as we
update them, we should use one that remains unchanged.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24139 fixed
a failure and introduced a new image. Let's use one that we already
use in the docs and we're unlikely to change its name.

doc/user/markdown.md

@@ -896,6 +896,11 @@ Reference-style (hover to see title text):
 [logo]: img/markdown_logo.png "Title Text"
 ```
 
+<!--
+DO NOT change the name of markdown_logo.png. This is used for a test
+in spec/controllers/help_controller_spec.rb.
+-->
+
 Inline-style (hover to see title text):
 
 ![alt text](img/markdown_logo.png "Title Text")

spec/controllers/help_controller_spec.rb

@@ -111,7 +111,7 @@ describe HelpController do
         it 'renders the raw file' do
           get :show,
               params: {
-                path: 'fixtures/gitlab_tanuki'
+                path: 'user/img/markdown_logo'
               },
               format: :png
           expect(response).to be_successful