Fix black Win32 smoke broken by my v5.21.1-11-g4077a6b (yet again)

I was being silly with my test improvements in v5.21.1-12-g826af13 and
v5.21.1-13-g43d7f0d. I haven't been able to test this on Win32 where
it's breaking, but if the sprintf format isn't recognized there we're
going to get a warning about a *redundant* argument in printf, not a
missing one, i.e. just like this:

    $ ./perl -Ilib -we 'printf "%æ\n", "str"'
    Invalid conversion in printf: "%\303" at -e line 1.
    Redundant argument in printf at -e line 1.
    %s

So fix that, I'm certain this will fix the black smoke on Win32, if not
I have no idea what's going on.