Specs for where $_ is defined and when it is defined?

These are core Ruby specs and should not depend on the standard library. I know you didn't add the stringio require, but part of working on rubyspec is fixing bad style or specs if you have to touch them or avoiding bad practices. We have simple helpers for creating and cleaning up files to test IO-related behavior. Besides not depending on standard library, StringIO is especially problematic because there have been numerous bugs in it over time. If we're spec'ing IO behavior, it needs to be with actual IO instances.