Add cop for modifying Sidekiq middleware in specs

This should prevent us from leaking sidekiq middleware across
specs. By default they should all be enabled.

For the specs that test 1 middleware specifically, we can use a helper.

rubocop/cop/rspec/modify_sidekiq_middleware.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module RSpec
+      # This cop checks for `Sidekiq::Testing.server_middleware`
+      # usage in specs.
+      #
+      # @example
+      #
+      #   # bad
+      #   Sidekiq::Testing.server_middleware do |chain|
+      #     chain.add(MyMiddlewareUnderTest)
+      #   end
+      #
+      #
+      #   # good
+      #   with_custom_sidekiq_middleware do |chain|
+      #     chain.add(MyMiddlewareUnderTest)
+      #   end
+      #
+      #
+      class ModifySidekiqMiddleware < RuboCop::Cop::Cop
+        MSG = <<~MSG
+          Don't modify global sidekiq middleware, use the `#with_sidekiq_server_middleware`
+          helper instead
+        MSG
+
+        def_node_search :modifies_sidekiq_middleware?, <<~PATTERN
+          (send
+            (const
+              (const nil? :Sidekiq) :Testing) :server_middleware)
+        PATTERN
+
+        def on_send(node)
+          return unless modifies_sidekiq_middleware?(node)
+
+          add_offense(node, location: :expression)
+        end
+
+        def autocorrect(node)
+          -> (corrector) do
+            corrector.replace(node.loc.expression,
+                              'with_sidekiq_server_middleware')
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/rspec/modify_sidekiq_middleware_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+require 'rubocop'
+require_relative '../../../support/helpers/expect_offense'
+require_relative '../../../../rubocop/cop/rspec/modify_sidekiq_middleware'
+
+describe RuboCop::Cop::RSpec::ModifySidekiqMiddleware do
+  include CopHelper
+  include ExpectOffense
+
+  subject(:cop) { described_class.new }
+
+  let(:source) do
+    <<~SRC
+    Sidekiq::Testing.server_middleware do |chain|
+      chain.add(MyCustomMiddleware)
+    end
+    SRC
+  end
+
+  let(:corrected) do
+    <<~SRC
+    with_sidekiq_server_middleware do |chain|
+      chain.add(MyCustomMiddleware)
+    end
+    SRC
+  end
+
+  it 'registers an offence' do
+    inspect_source(source)
+
+    expect(cop.offenses.size).to eq(1)
+  end
+
+  it 'can autocorrect the source' do
+    expect(autocorrect_source(source)).to eq(corrected)
+  end
+end