meson: Mark gdatetime test as slow
meson: Mark gdatetime test as slow

This test isn't inherently slow, but it produces so much output that
it can take a minute or more on hardware with weak I/O performance.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib