Use the test-unit-ruby-core gem for Test::Unit::CoreAssertions

test/lib/core_assertions.rb and its dependencies are now packaged in
the gem test-unit-ruby-core. Let's use it instead.

test/openssl/utils.rb

@@ -5,15 +5,10 @@
 end
 
 require "test/unit"
+require "core_assertions"
 require "tempfile"
 require "socket"
 
-begin
-  require_relative "../lib/core_assertions"
-rescue LoadError
-  # for ruby/ruby repository
-end
-
 if defined?(OpenSSL)
 
 module OpenSSL::TestUtils