gtestutils: fix g_test_set_nonfatal_assertions()

gtestutils: fix g_test_set_nonfatal_assertions()

g_test_set_nonfatal_assertions() was a no-op, because
g_assertion_message() wasn't actually checking the
test_nonfatal_assertions flag. Fix that and add a test.

Also, g_test_set_nonfatal_assertions() has to set test_mode_fatal to
FALSE as well, or else a failed assertion will cause the test program
to abort at the end of the failed test.

Also, belatedly add this and the new g_assert_* methods to the docs.

https://bugzilla.gnome.org/show_bug.cgi?id=711800