test_string.rb: move and add assertions

* test/ruby/test_string.rb (test_slice): move the assertion for
  [Bug #9882], and more assertion for `ascii_only?` too, as
  pointed out by Heesob Park in [ruby-core:62846].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46244 b2dd03c8-39d4-4d8f-98ff-823fe69b080e