Move core_assertions.rb from test/unit

This file contains extended assertions for ruby core which do not
belong to test/unit.

tool/lib/core_assertions.rb

@@ -25,7 +25,7 @@ module Test
 
     module CoreAssertions
       if defined?(MiniTest)
-        require_relative '../../envutil'
+        require_relative 'envutil'
         # for ruby core testing
         include MiniTest::Assertions
 

tool/lib/test/unit/assertions.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 require 'minitest/unit'
-require 'test/unit/core_assertions'
+require_relative '../../core_assertions'
 require 'pp'
 
 module Test