Add a smoke test, running yard on the full yard source

As this will take a significant fraction of the running time of the
whole test suite (roughly 4x more than the rest), we isolate this
test in its own file.

Therefore, one can run the whole test suite as before with:

  bundler exec rake spec

Or the whole test suite except this smoke test:

  rspec --exclude-pattern spec/cli/yard_on_yard_spec.rb

spec/cli/yard_on_yard_spec.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+$TOPDIR = File.expand_path(File.join(File.dirname(__FILE__), '../..'))
+
+require 'fileutils'
+
+RSpec.describe YARD::CLI::Yardoc do
+  include FileUtils
+
+  context 'building the documentation of YARD itself' do
+    before(:context) do
+      rm_rf File.join($TOPDIR, 'doc')
+      rm_rf File.join($TOPDIR, '.yardoc')
+
+      # Note: as this is very time consuming, we do it only once
+      Dir.chdir($TOPDIR) do
+        @res = YARD::CLI::Yardoc.new.run('--title', 'YARD-On-YARD')
+      end
+    end
+
+    it 'succeeds and creates doc/ and .yardoc/' do
+      expect(@res).to be true
+      expect(Dir.exist?('doc')).to be true
+      expect(Dir.exist?('.yardoc')).to be true
+    end
+
+    it 'writes the given title in each documentation file' do
+      Dir.glob(File.join($TOPDIR, 'doc/**/*.html')) do |htmlfile|
+        next if %w(
+          frames file_list class_list method_list tag_list _index
+        ).include?(File.basename(htmlfile, '.html'))
+        html = File.read(htmlfile)
+
+        expect(html.index('— YARD-On-YARD')).to be >= 0
+      end
+    end
+  end
+end