scx: Fix exit selftest to use custom DSQ

In commit 63fb3ec8 ("sched_ext: Allow only user DSQs for
scx_bpf_consume(), scx_bpf_dsq_nr_queued() and bpf_iter_scx_dsq_new()"), we
updated the consume path to only accept user DSQs, thus making it invalid
to consume SCX_DSQ_GLOBAL. This selftest was doing that, so let's create a
custom DSQ and use that instead.  The test now passes:

[root@virtme-ng sched_ext]# ./runner -t exit
===== START =====
TEST: exit
DESCRIPTION: Verify we can cleanly exit a scheduler in multiple places
OUTPUT:
[   12.387229] sched_ext: BPF scheduler "exit" enabled
[   12.406064] sched_ext: BPF scheduler "exit" disabled (unregistered from BPF)
[   12.453325] sched_ext: BPF scheduler "exit" enabled
[   12.474064] sched_ext: BPF scheduler "exit" disabled (unregistered from BPF)
[   12.515241] sched_ext: BPF scheduler "exit" enabled
[   12.532064] sched_ext: BPF scheduler "exit" disabled (unregistered from BPF)
[   12.592063] sched_ext: BPF scheduler "exit" disabled (unregistered from BPF)
[   12.654063] sched_ext: BPF scheduler "exit" disabled (unregistered from BPF)
[   12.715062] sched_ext: BPF scheduler "exit" disabled (unregistered from BPF)
ok 1 exit #
=====  END  =====

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>