test_benchmark.rb: use assert_in_epsilon

* test/benchmark/test_benchmark.rb (test_realtime_output): use
  assert_in_epsilon which compares in relative range, instead of
  assert_in_delta which compares in absolute range.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47329 b2dd03c8-39d4-4d8f-98ff-823fe69b080e