scx_qmap: Implement highpri boosting

Implement a silly boosting mechanism for nice -20 tasks. The only purpose is
demonstrating and testing scx_bpf_dispatch_from_dsq(). The boosting only
works within SHARED_DSQ and makes only minor differences with increased
dispatch batch (-b).

This exercises moving tasks to a user DSQ and all local DSQs from
ops.dispatch() and BPF timerfn.

v2: - Updated to use scx_bpf_dispatch_from_dsq_set_{slice|vtime}().

    - Drop the workaround for the iterated tasks not being trusted by the
      verifier. The issue is fixed from BPF side.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Daniel Hodges <hodges.daniel.scott@gmail.com>
Cc: David Vernet <void@manifault.com>
Cc: Changwoo Min <multics69@gmail.com>
Cc: Andrea Righi <andrea.righi@linux.dev>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>