[ruby/rdoc] Try to load WEBrick for testing

This is in similar vein Rake is optionally loaded in:

5437418baf/test/rdoc/test_rdoc_task.rb (L3-L6)

https://github.com/ruby/rdoc/commit/5f3901ae60

test/rdoc/test_rdoc_servlet.rb

@@ -1,5 +1,9 @@
 # frozen_string_literal: true
 require_relative 'helper'
+begin
+  require 'webrick'
+rescue LoadError
+end
 
 class TestRDocServlet < RDoc::TestCase