tests: Make it easier to keep track of where we are

The monitor orientation tests do a lot of things in sequence. Replace
some of the comments with g_test_message() so that the log from a failed
test gives us a better idea of how far we got.

Signed-off-by: Simon McVittie <smcv@debian.org>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2049>
(cherry picked from commit 838d5656)