libqtest: always set up signal handler for SIGABRT

Currently abort handlers only work for the first test function
in a testcase, because the list of abort handlers is not properly
cleared when qtest_quit() is called.

qtest_quit() only deletes the kill_qemu_hook but doesn't completely
clear the abrt_hooks list.  The effect is that abrt_hooks.is_setup is
never set to false and in a following test the abrt_hooks list is not
initialized and setup_sigabrt_handler() is not called.

One way to solve this is to clear the list in qtest_quit(), but
that means only asserts between qtest_start and qtest_quit will
be catched by the abort handler.

We can make abort handlers work in all cases if we always setup the
signal handler for SIGABRT in qtest_init.

Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
