testutils: Add a backport of g_test_disable_crash_reporting()
When testing something that is expected to fail or crash, it's useful to disable core dump reporting.
This is a slightly simplified version of glib!3510 (merged): because libglnx isn't portable to non-Linux, we can assume that <sys/prctl.h>, prctl() and <sys/resource.h> are always available, so we don't need to check for them in the build system.
/cc @pwithnall @mcatanzaro since you liked this so much in GLib