test_string.rb: Bug #8642

* test/ruby/test_string.rb (test_split): test for r41994.
  [ruby-core:56036] [Bug #8642]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42002 b2dd03c8-39d4-4d8f-98ff-823fe69b080e