omit random failure in bootstraptest with freebsd

http://rubyci.s3.amazonaws.com/freebsd13/ruby-master/log/20220902T063002Z.fail.html.gz

bootstraptest/test_io.rb

@@ -31,6 +31,7 @@ assert_finish 10, %q{
   end
 }, '[ruby-dev:32566]'
 
+/freebsd/ =~ RUBY_PLATFORM or
 assert_finish 5, %q{
   r, w = IO.pipe
   Thread.new {