Backport changes related to extracted stdlibs in Ruby 3.4-3.5

[ This is a backport to the 3.1 branch. ]

This is a squash of the following commits, and brings the content of
Gemfile to match master:

  b0acc1a Make optional benchmark test in OpenSSL::OSSL#test_memcmp_timing
  4312b07 Add rdoc as a development dependency.
  f59ec58 Use the test-unit-ruby-core gem for Test::Unit::CoreAssertions
  459f20b Add prime gem to development dependency

Author: rhenium

Gemfile

@@ -4,4 +4,8 @@ group :development do
   gem "rake"
   gem "rake-compiler"
   gem "test-unit", "~> 3.0", ">= 3.4.6"
+  gem "test-unit-ruby-core"
+  gem "prime"
+  # In the case of Ruby whose rdoc is not a default gem.
+  gem "rdoc"
 end

test/openssl/test_ossl.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 require_relative "utils"
 
-require 'benchmark'
-
 if defined?(OpenSSL)
 
 class OpenSSL::OSSL < OpenSSL::SSLTestCase
@@ -44,6 +42,12 @@ def test_secure_compare
   end
 
   def test_memcmp_timing
+    begin
+      require "benchmark"
+    rescue LoadError
+      pend "Benchmark is not available in this environment. Please install it with `gem install benchmark`."
+    end
+
     # Ensure using fixed_length_secure_compare takes almost exactly the same amount of time to compare two different strings.
     # Regular string comparison will short-circuit on the first non-matching character, failing this test.
     # NOTE: this test may be susceptible to noise if the system running the tests is otherwise under load.