DEV: Connecting to external URLs causes tests to hang (#96)

Using an external URL was causing the browser to actually attempt to
connect to it. Sometimes, a response from the external URL will take
more than 30 seconds to return a response causing the page to never fire
the load event before playwright's timeout is reached.

spec/system/image_signatures_spec.rb

@@ -4,7 +4,7 @@ RSpec.describe "Image signatures", type: :system do
   fab!(:user)
   fab!(:topic) { Fabricate(:topic, category: Fabricate(:category)) }
   fab!(:post) { Fabricate(:post, topic:) }
-  let(:signature_image_url) { "https://example.com/signature.png" }
+  let(:signature_image_url) { "data:abcdef," }
 
   context "when signatures plugin is enabled" do
     before do