test_io.rb: use assert_not_predicate
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56432 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_io.rb: use assert_not_predicate
for more descriptive message.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56432 b2dd03c8-39d4-4d8f-98ff-823fe69b080e