selftests/sched_ext: Validate dl_server attach/detach in total_bw test

selftests/sched_ext: Validate dl_server attach/detach in total_bw test

Extend the total_bw selftest to validate the fair/ext dl_server
auto-attach/detach operations.

After the existing consistency checks, the test now doubles the
fair_server's runtime on every CPU via debugfs and verifies that:
 1. total_bw grew after the customization (proves fair_server was
    attached and apply_params() honored the dl_bw_attached flag),
 2. with the minimal BPF scheduler loaded, total_bw drops back to the
    baseline value (proves fair_server was detached and ext_server was
    attached at its own default runtime),
 3. after unload total_bw matches the doubled value from step 1 (proves
    fair_server was re-attached with the runtime customization preserved
    across the load/unload cycle).

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://patch.msgid.link/20260526164420.638711-3-arighi@nvidia.com