Test ArraySerializer less rigorously on Minitest 4

lib/active_model/serializer/array_serializer.rb

@@ -2,7 +2,7 @@ require 'active_model/serializer/collection_serializer'
 class ActiveModel::Serializer
   class ArraySerializer < CollectionSerializer
     def initialize(*)
-      warn "Calling deprecated ArraySerializer in #{caller[0]}. Please use CollectionSerializer"
+      warn "Calling deprecated ArraySerializer in #{caller[0..2].join(', ')}. Please use CollectionSerializer"
       super
     end
   end

test/array_serializer_test.rb

@@ -3,6 +3,8 @@ require_relative 'collection_serializer_test'
 
 module ActiveModel
   class Serializer
+    # Minitest.run_one_method isn't present in minitest 4
+    if $minitest_version > 4 # rubocop:disable Style/GlobalVars
       class ArraySerializerTest < CollectionSerializerTest
         extend ActiveSupport::Testing::Stream
         def self.run_one_method(*)
@@ -18,5 +20,19 @@ module ActiveModel
           ArraySerializer
         end
       end
+    else
+      class ArraySerializerTest < Minitest::Test
+        extend ActiveSupport::Testing::Stream
+        def test_json_key_with_root_warns_when_using_array_serializer
+          stderr = (capture(:stderr) do
+            comment = Comment.new
+            post = Post.new
+            serializer = ArraySerializer.new([comment, post])
+            assert_equal serializer.json_key, 'comments'
+          end)
+          assert_match(/Calling deprecated ArraySerializer/, stderr)
+        end
+      end
+    end
   end
 end

test/test_helper.rb

@@ -25,10 +25,12 @@ require 'minitest/autorun'
 require 'minitest/reporters'
 Minitest::Reporters.use!
 if defined?(Minitest::Test)
+  $minitest_version = 5 # rubocop:disable Style/GlobalVars
   # Minitest 5
   # https://github.com/seattlerb/minitest/blob/e21fdda9d/lib/minitest/autorun.rb
   # https://github.com/seattlerb/minitest/blob/e21fdda9d/lib/minitest.rb#L45-L59
 else
+  $minitest_version = 4 # rubocop:disable Style/GlobalVars
   # Minitest 4
   # https://github.com/seattlerb/minitest/blob/644a52fd0/lib/minitest/autorun.rb
   # https://github.com/seattlerb/minitest/blob/644a52fd0/lib/minitest/unit.rb#L768-L787