test_sleep.rb: get rid of failure on Windows

* test/ruby/test_sleep.rb (test_sleep_5sec): relax bottom on Windows.
  assert_operator should test on the receiver.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43153 b2dd03c8-39d4-4d8f-98ff-823fe69b080e