* test/dtrace/helper.rb: ignore empty SUDO.  reuse SUDO value by using
  class variable.  use test/unit instead of minitest/autorun.  dispose
  `Interrupted system call' error messages at parallel test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40630 b2dd03c8-39d4-4d8f-98ff-823fe69b080e