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
gtestutils: add g_assert_nonnull() to go with g_assert_null()

https://bugzilla.gnome.org/show_bug.cgi?id=711800
Reorganize the "don't dump core from test subprocesses" code.

g_test_init() was calling _g_messages_set_exit_on_fatal() from
subprocesses, to make fatal log messages call _exit() rather than
abort(), but the function name is sort of confusing, and we don't
really need it anyway, since g_log() can just call g_test_subprocess()
instead and decide for itself.

Likewise, update g_assertion_message() to do the check itself, rather
than calling into gmessages to do it, and fix
g_assertion_message_expr() to also check whether it should exit or
abort. (Previously it always called abort(), although this didn't
actually matter since that was dead code until
test_nonfatal_assertions was added.)

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