gtestutils: Mention G_DISABLE_ASSERT in documentation for g_test_init()

g_test_init() bails if G_DISABLE_ASSERT is defined. Mention that. This
is a follow-up to
GNOME/glib!174.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1446
8 jobs for 1446-g_test_init-docs in 13 minutes and 44 seconds (queued for 7 seconds)
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