Add Equivalent Windows OS Tests

[danielpclark]

Many of the tests already have been included for the Windows operating system. This issue is to check the tests against the pathname test source & file exhaustive test and add in any that are missing. This way we can make confident progress on Windows support.

Checklist

• absolute?
• add_trailing_separator
• basename
• children
• children_compat
• chop_basename (no tests on official Ruby)
• cleanpath_aggressive
• cleanpath_conservative
• del_trailing_separator
• directory?
• dirname
• entries
• entries_compat
• extname
• has_trailing_separator?
• join
• plus
• relative?
• relative_path_from

This Issue is just for adding the tests — not to make them pass.

POST

• Open issues per failing Windows scenario