Use test-unit instead of minitest

Minitest 6 will err `assert_equal` with `nil`.
minitest/minitest#779

Author: nobu

lib/rdoc/markup/formatter_test_case.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require 'minitest/unit'
+require 'test-unit'
 
 ##
 # Test case for creating new RDoc::Markup formatters.  See

test/minitest_helper.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 begin
-  gem 'minitest', '~> 5.0'
+  gem 'test-unit'
 rescue NoMethodError, Gem::LoadError
   # for ruby tests
 end
 
-require 'minitest/autorun'
-require 'minitest/benchmark' unless ENV['NOBENCHMARK']
+require 'test-unit'
+# require 'minitest/benchmark' unless ENV['NOBENCHMARK']
 
 require 'fileutils'
 require 'pp'
@@ -29,7 +29,7 @@
 # * <code>@pwd</code> containing the current working directory
 # * FileUtils, pp, Tempfile, Dir.tmpdir and StringIO
 
-class RDoc::TestCase < (defined?(Minitest::Test) ? Minitest::Test : MiniTest::Unit::TestCase)
+class RDoc::TestCase < Test::Unit::TestCase
 
   ##
   # Abstract test-case setup
@@ -190,7 +190,7 @@ def verb *parts
   # run capture_io with setting $VERBOSE = true
 
   def verbose_capture_io
-    capture_io do
+    capture_output do
       begin
         orig_verbose = $VERBOSE
         $VERBOSE = true
@@ -200,4 +200,7 @@ def verbose_capture_io
       end
     end
   end
+
+  alias capture_io capture_output
+  alias skip omit
 end