test: verify referenced sponsor images are available

Author: salzig

app/assets/images/labels/tallinn_rug/sponsors/rubycentral.png

@@ -569,3 +569,4 @@
   sponsors:
   - :name: RubyCentral
     :url: https://rubycentral.org/
+    :banner: rubycentral.png

config/whitelabel.yml

@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+describe 'Sponsors' do
+  Whitelabel.labels.each do |label|
+    describe "for '#{label.label_id}'" do
+      label.sponsors&.each do |sponsor|
+        specify sponsor[:banner] do
+          expect(Rails.root.join('app/assets/images/labels').join(label.label_id).join('sponsors').join(sponsor[:banner])).to be_exist
+        end
+      end
+    end
+  end
+end