tests: arch: arm_thread_swap: minor fixes

Some fixes to the ARM Thread Swap test:
- replace a mempcy with memset (this did not have an
  effect, as we basically wanted to randomize the
  callee-saved registers)
- fix two inline comments so they correspond to the
  z_assert_ expressions.

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