Fix leaking constant

rubocop/rubocop#8095 allowed to dynamically
declare cops with Class.new

We don't have to bump minimally required RuboCop version since it only
affects our specs, and we always use the latest RuboCop for specs.

spec/rubocop/cop/rspec/cop_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::Cop::RSpec::Cop do
-  subject(:cop) { fake_cop.new(config) }
+  subject(:cop) { RuboCop::RSpec::FakeCop.new(config) }
 
   let(:config) do
     rubocop_config =
@@ -19,21 +19,17 @@
     RuboCop::Config.new(rubocop_config, 'fake_cop_config.yml')
   end
 
-  let(:fake_cop) do
-    # rubocop:disable Style/ClassAndModuleChildren
-    # rubocop:disable RSpec/LeakyConstantDeclaration
-    class RuboCop::RSpec::FakeCop < described_class
-      def on_send(node)
-        add_offense(node, message: 'I flag everything')
-      end
-    end
-    # rubocop:enable Style/ClassAndModuleChildren
-    # rubocop:enable RSpec/LeakyConstantDeclaration
-    RuboCop::RSpec::FakeCop
-  end
-
   let(:rspec_patterns) { ['_spec.rb$', '(?:^|/)spec/'] }
 
+  before do
+    stub_const('RuboCop::RSpec::FakeCop',
+               Class.new(described_class) do
+                 def on_send(node)
+                   add_offense(node, message: 'I flag everything')
+                 end
+               end)
+  end
+
   context 'when the source path ends with `_spec.rb`' do
     it 'registers an offense' do
       expect_offense(<<-RUBY, 'foo_spec.rb')