print arena stats only when running test_suite --verbose

i was trying to find out why the test suite suddenly insisted on being so
noisy. turns out glib now does TAP by default and all messages get printed,
--verbose or not. the old mode is considered deprecated.

cf.
https://en.wikipedia.org/wiki/Test_Anything_Protocol
https://gitlab.gnome.org/GNOME/glib/commit/ad175766575b23be920f41cdb1c350447c743347
https://gitlab.gnome.org/GNOME/glib/commit/19aafc4ca4dba2b63d24f01d23890ff30ef9b133