RSpec helpers: Extract duplicated code

expect_next_instance_of and allow_next_instance_of use the same code
expect the target (expect or allow).

spec/spec_helper.rb

@@ -90,8 +90,7 @@ RSpec.configure do |config|
   config.include StubFeatureFlags
   config.include StubGitlabCalls
   config.include StubGitlabData
-  config.include AllowNextInstanceOf
-  config.include ExpectNextInstanceOf
+  config.include NextInstanceOf
   config.include TestEnv
   config.include Devise::Test::ControllerHelpers, type: :controller
   config.include Devise::Test::IntegrationHelpers, type: :feature

spec/support/helpers/allow_next_instance_of.rb

-# frozen_string_literal: true
-
-module AllowNextInstanceOf
-  def allow_next_instance_of(klass, *new_args)
-    receive_new = receive(:new)
-    receive_new.with(*new_args) if new_args.any?
-
-    allow(klass).to receive_new
-      .and_wrap_original do |method, *original_args|
-        method.call(*original_args).tap do |instance|
-          yield(instance)
-        end
-      end
-  end
-end

spec/support/helpers/expect_next_instance_of.rb

-# frozen_string_literal: true
-
-module ExpectNextInstanceOf
-  def expect_next_instance_of(klass, *new_args)
-    receive_new = receive(:new)
-    receive_new.with(*new_args) if new_args.any?
-
-    expect(klass).to receive_new
-      .and_wrap_original do |method, *original_args|
-        method.call(*original_args).tap do |instance|
-          yield(instance)
-        end
-      end
-  end
-end

spec/support/helpers/next_instance_of.rb

+# frozen_string_literal: true
+
+module NextInstanceOf
+  def expect_next_instance_of(klass, *new_args)
+    stub_new(expect(klass), *new_args) do |expectation|
+      yield(expectation)
+    end
+  end
+
+  def allow_next_instance_of(klass, *new_args)
+    stub_new(allow(klass), *new_args) do |allowance|
+      yield(allowance)
+    end
+  end
+
+  private
+
+  def stub_new(target, *new_args)
+    receive_new = receive(:new)
+    receive_new.with(*new_args) if new_args.any?
+
+    target.to receive_new.and_wrap_original do |method, *original_args|
+      method.call(*original_args).tap do |instance|
+        yield(instance)
+      end
+    end
+  end
+end