Document difference between g_assert() and g_assert_*() wrt G_DISABLE_ASSERT

This adds documentation to clarify the difference between g_assert() and g_assert_*(), and also implements Dan’s suggestion of overriding g_test_init() when G_DISABLE_ASSERT is defined, to abort tests.

I did think that not many modules use G_DISABLE_ASSERT, but actually grepping through my GNOME git repositories, quite a few do. @ebassi, I think this is something to (review and then, if accepted) try out on Continuous and see what issues arise; and potentially revert if lots of problems appear. The documentation should be good regardless though.

Closes #976 (closed)