tests: arm_thread_swap_tz: increase computational length for secure call

In some platforms, such as the nucleo L552ZE_Q, the hash computation
is done very fast, before the delayed work manages to trigger (and
perform the assert-based validations). Increase the length of the
computations so this race condition can be avoided.

Also, this commit adds some messages in the assert statements for
ease of debugging.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>