tests: replace fprintf() with testlog()

tests: replace fprintf() with testlog()

When we move on to TAP, stdout will be reserved for TAP and stderr is for free
chatter. Set up an example that tests should use testlog() instead of fprintf
or printf to chat in the right place.

Most statements were already printing to stderr, so this just makes then a
little shorter. There are also some statements that printed to stdout and are
now corrected.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>