Use IPC::Run3 for portability and test with a fake spell check cmd

Would you consider switching from IPC::Open3 to IPC::Run3?
The code to use it is much simpler and it actually works on windows (unlike everything else I tried).

Test::Spelling 0.12 worked on windows, and I believe that 0.13 broke this when it attempted reading STDERR.  IPC::Open3 documents that reading from both STDOUT and STDERR will cause you to want to kill yourself (ok, I am paraphrasing), but doesn't offer any solutions.

I also added a test file that uses perl to create a fake spell check command so that the module's functionality can be tested for portability even if no real spell checker is found.

I started looking into this as a response to
https://rt.cpan.org/Ticket/Display.html?id=82173#txn-1169276
and
https://github.com/xenoterracide/Dist-Zilla-Plugin-Test-PodSpelling/issues/7

What do you think?