test: improve path tests

This commit adds new tests, executes tests for other platforms
instead of limiting platform-specific tests to those platforms,
and fixes a few style/formatting inconsistencies.

PR-URL: https://github.com/nodejs/node/pull/5123


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>