work harder to get useful diagnostics on Win32

The pod2html tests can do diffs of big hunks, and so they try to
use /usr/bin/diff to show how things differ.  If there is no such
program, we just say "it wasn't good."

Unfortunately, Win32 is where we keep having problems, and where
we are least likely to have diff available.  When there is no diff,
we now use Test::More::is, which will output the whole differing file.

Noisy, yes, but now smoke reports with failing Pod::Html tests will
be more useful.

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