Follow up to #1535

- The removed classes and modules were added back with deprecation
  warning and deprecation test were added for them.
- One test was renamed because it contained `__`.
- Some tests were refactored.
- The ActiveModelSerializers::Deserialization module is now called
  Adapter instead of ActiveModelSerializers::Adapter.
- The changelog was added for #1535

test/adapter/deprecation_test.rb

@@ -0,0 +1,109 @@
+require 'test_helper'
+module ActiveModel
+  class Serializer
+    module Adapter
+      class DeprecationTest < ActiveSupport::TestCase
+        class DeprecatedPostSerializer < ActiveModel::Serializer
+          attribute :body
+        end
+        setup do
+          post = Post.new(id: 1, body: 'Hello')
+          @serializer = DeprecatedPostSerializer.new(post)
+        end
+
+        def test_null_adapter_serialization
+          assert_equal({}, Null.new(@serializer).as_json)
+        end
+
+        def test_json_adapter_serialization
+          assert_equal({ post: { body: 'Hello' } }, Json.new(@serializer).as_json)
+        end
+
+        def test_jsonapi_adapter_serialization
+          expected = {
+            data: {
+              id: '1',
+              type: 'posts',
+              attributes: {
+                body: 'Hello'
+              }
+            }
+          }
+          assert_equal(expected, JsonApi.new(@serializer).as_json)
+        end
+
+        def test_attributes_adapter_serialization
+          assert_equal({ body: 'Hello' }, Attributes.new(@serializer).as_json)
+        end
+
+        def test_null_adapter_deprecation
+          assert_deprecated_adapter(Null)
+        end
+
+        def test_json_adapter_deprecation
+          assert_deprecated_adapter(Json)
+        end
+
+        def test_json_api_adapter_deprecation
+          assert_deprecated_adapter(JsonApi)
+        end
+
+        def test_attributes_adapter_deprecation
+          assert_deprecated_adapter(Attributes)
+        end
+
+        def test_adapter_create_deprecation
+          assert_deprecated do
+            Adapter.create(@serializer)
+          end
+        end
+
+        def test_adapter_adapter_map_deprecation
+          assert_deprecated do
+            Adapter.adapter_map
+          end
+        end
+
+        def test_adapter_adapters_deprecation
+          assert_deprecated do
+            Adapter.adapters
+          end
+        end
+
+        def test_adapter_adapter_class_deprecation
+          assert_deprecated do
+            Adapter.adapter_class(:json_api)
+          end
+        end
+
+        def test_adapter_register_deprecation
+          assert_deprecated do
+            Adapter.register(:test, Class.new)
+            Adapter.adapter_map.delete('test')
+          end
+        end
+
+        def test_adapter_lookup_deprecation
+          assert_deprecated do
+            Adapter.lookup(:json_api)
+          end
+        end
+
+        private
+
+        def assert_deprecated_adapter(adapter)
+          assert_deprecated do
+            adapter.new(@serializer)
+          end
+        end
+
+        def assert_deprecated
+          message = /deprecated/
+          assert_output(nil, message) do
+            yield
+          end
+        end
+      end
+    end
+  end
+end