scx: Fix maximal BPF selftest prog

maximal.bpf.c is still dispatching to and consuming from SCX_DSQ_GLOBAL.
Let's have it use its own DSQ to avoid any runtime errors.

Signed-off-by: David Vernet <void@manifault.com>
Tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>