Add a basic test for the docs generator

Nothing much, just checking that it runs without errors on all ruby versions tested in CI

Followup to #13332

Author: Earlopain

lib/rubocop/cops_documentation_generator.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'fileutils'
+require 'yard'
 
 # Class for generating documentation of all cops departments
 # @api private
@@ -295,7 +296,7 @@ def print_cops_of_department(department)
     HEADER
     selected_cops.each { |cop| content << print_cop_with_doc(cop) }
     content << footer_for_department(department)
-    file_name = "#{docs_path}/#{department_to_basename(department)}.adoc"
+    file_name = "#{docs_path}#{department_to_basename(department)}.adoc"
     File.open(file_name, 'w') do |file|
       puts "* generated #{file_name}"
       file.write("#{content.strip}\n")

spec/rubocop/cops_documentation_generator_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require 'rubocop/cops_documentation_generator'
+
+RSpec.describe CopsDocumentationGenerator, :isolated_environment do
+  around do |example|
+    new_global = RuboCop::Cop::Registry.new([RuboCop::Cop::Style::HashSyntax])
+    RuboCop::Cop::Registry.with_temporary_global(new_global) { example.run }
+  end
+
+  it 'generates docs without errors' do
+    generator = described_class.new(departments: %w[Style])
+    expect do
+      generator.call
+    end.to output(%r{generated .*docs/modules/ROOT/pages/cops_style.adoc}).to_stdout
+  end
+end