test_rand.rb: refine probability test

• test/ruby/test_rand.rb (test_new_seed): refine probability of
  failure from 1/256 to 1/256**8.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55955 b2dd03c8-39d4-4d8f-98ff-823fe69b080e