Explicitly set github repo in instance presenter spec (#24036)

Gemfile

@@ -117,6 +117,7 @@ end
 
 group :test do
   gem 'capybara', '~> 3.38'
+  gem 'climate_control'
   gem 'faker', '~> 3.1'
   gem 'json-schema', '~> 3.0'
   gem 'rack-test', '~> 2.0'

Gemfile.lock

@@ -784,6 +784,7 @@ DEPENDENCIES
   capybara (~> 3.38)
   charlock_holmes (~> 0.7.7)
   chewy (~> 7.2)
+  climate_control
   cocoon (~> 1.2)
   color_diff (~> 0.1)
   concurrent-ruby

spec/presenters/instance_presenter_spec.rb

@@ -89,10 +89,30 @@ describe InstancePresenter do
   end
 
   describe '#source_url' do
-    it 'returns "https://github.com/mastodon/mastodon"' do
+    context 'with the GITHUB_REPOSITORY env variable set' do
+      around do |example|
+        ClimateControl.modify GITHUB_REPOSITORY: 'other/repo' do
+          example.run
+        end
+      end
+
+      it 'uses the env variable to build a repo URL' do
+        expect(instance_presenter.source_url).to eq('https://github.com/other/repo')
+      end
+    end
+
+    context 'without the GITHUB_REPOSITORY env variable set' do
+      around do |example|
+        ClimateControl.modify GITHUB_REPOSITORY: nil do
+          example.run
+        end
+      end
+
+      it 'defaults to the core mastodon repo URL' do
         expect(instance_presenter.source_url).to eq('https://github.com/mastodon/mastodon')
       end
     end
+  end
 
   describe '#thumbnail' do
     it 'returns SiteUpload' do