Use test/unit instead of test-unit. Because test-unit is only provided standalone gem.

lib/rdoc/markup/formatter_test_case.rb

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

test/rdoc/helper.rb

@@ -6,7 +6,7 @@ rescue NoMethodError, Gem::LoadError, Bundler::GemfileNotFound
   # for ruby tests
 end
 
-require 'test-unit'
+require 'test/unit'
 
 require 'fileutils'
 require 'pp'