Add `#to_s` methods to aid debugging

lib/gitlab/ci/variables/collection.rb

@@ -95,6 +95,10 @@ module Gitlab
           new_collection
         end
 
+        def to_s
+          "#{@variables_by_key.keys}, @errors='#{@errors}'"
+        end
+
         protected
 
         attr_reader :variables

lib/gitlab/ci/variables/collection/item.rb

@@ -63,6 +63,12 @@ module Gitlab
               raise ArgumentError, "Unknown `#{resource.class}` variable resource!"
             end
           end
+
+          def to_s
+            return to_runner_variable.to_s unless depends_on
+
+            "#{to_runner_variable}, depends_on=#{depends_on}"
+          end
         end
       end
     end