tests: sched: Add busy threads for SMP

The sched benchmark is designed for systems with a single
CPU. Otherwise, the timestamps would be wrong when the partner
thread is scheduled on another CPU, i.e. negative values:

```
unpend   63 ready   62 switch -16562 pend 18937 tot 2500 (avg  928)
```

When the system allows for multiple CPUs, spawn a non-preemptible
thread to keep the other CPUs busy.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>