minitest is no longer a stdlib on any of the supported versions of Rubies

CRuby's bundled minitest had been removed in 2.2.0 at
ruby/ruby@7cda822

Author: amatsuda

activesupport/lib/active_support/test_case.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 
-gem "minitest" # make sure we get the gem, not stdlib
 require "minitest"
 require "active_support/testing/tagged_logging"
 require "active_support/testing/setup_and_teardown"

activesupport/lib/active_support/testing/autorun.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-gem "minitest"
-
 require "minitest"
 
 Minitest.autorun