rename test causing unpack errors on Windows

A file with colons in its name is causing unpack errors on Windows.
See https://rt.cpan.org/Ticket/Display.html?id=90339.  Rename the
offending file.
change README to README.pod

Instead of maintaining a separate readme, grub the readme from the
POD via `podselect lib/Test/Builder/Clutch.pm > README.pod`.