Add p5-Test-MockFile: Validate file-interaction code that without touching the file system

Test::MockFile - Allows tests to validate code that can interact with
files without touching the file system.

Intercepts file system calls for specific files so unit testing can take
place without any files being altered on disk.

This is useful for small tests where file interaction is discouraged.

A strict mode is even provided (and turned on by default) which can
throw a die when files are accessed during your tests!

(schmonz)