tests: latency_measure: fix stacks for the busy threads

The current `K_THREAD_STACK_DEFINE` only create a single stack
shared by all the busy threads. This is causing the application
to crash when there are more than 2 cores in the system.

We should use `K_THREAD_STACK_ARRAY_DEFINE` to create an array
of stacks instead.

Updated the testcase to test up to 8 cores using
qemu_riscv64_smp

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