tests: consistently use virFilePrintf

tests: consistently use virFilePrintf

Commit a2619962 introduced virFilePrintf to work around the fact
that gnulib doesn't (yet) provide guarantees about fprintf() vs.
%z, which in turn causes all sorts of mingw compilation errors:

../../tests/testutils.c: In function 'virtTestResult':
../../tests/testutils.c:101:9: error: unknown conversion type character 'z' in format [-Werror=format=]
         fprintf(stderr, "%3zu) %-60s ", testCounter, name);
         ^

Rather than s/fprintf/virFilePrintf/ (and reformatting loads of
lines) across multiple files, it's easier to just hack the entire
testsuite to take advantage of our helper function.

* tests/testutils.c: s/fprintf/virFilePrintf/ for mingw.

Signed-off-by: Eric Blake <eblake@redhat.com>