[ruby/io-wait] Use omit instead of skip for test-unit

https://github.com/ruby/io-wait/commit/75543ab1bc

test/io/wait/test_io_wait_uncommon.rb

@@ -13,7 +13,7 @@ class TestIOWaitUncommon < Test::Unit::TestCase
   end
 
   def test_fifo_wait
-    skip 'no mkfifo' unless File.respond_to?(:mkfifo) && IO.const_defined?(:NONBLOCK)
+    omit 'no mkfifo' unless File.respond_to?(:mkfifo) && IO.const_defined?(:NONBLOCK)
     require 'tmpdir'
     Dir.mktmpdir('rubytest-fifo') do |dir|
       fifo = "#{dir}/fifo"