Add spec

Test that usernames with underscores get linked correctly

Signed-off-by: Plastikmensch <plastikmensch@users.noreply.github.com>

spec/lib/advanced_text_formatter_spec.rb

@@ -75,6 +75,15 @@ RSpec.describe AdvancedTextFormatter do
         it 'creates a mention link' do
           expect(subject).to include '<a href="https://cb6e6126.ngrok.io/@alice" class="u-url mention">@<span>alice</span></a></span>'
         end
+
+        context 'when username contains underscores' do
+          let(:preloaded_accounts) { [Fabricate(:account, username: '_bob_')] }
+          let(:text) { '@_bob_' }
+
+          it 'creates a mention link' do
+            expect(subject).to include '<a href="https://cb6e6126.ngrok.io/@_bob_" class="u-url mention">@<span>_bob_</span></a></span>'
+          end
+        end
       end
 
       context 'with text containing unlinkable mentions' do