Test: Refactor IRQ Testing

This patch:
 - Replaces the tfm_irq_test_service_1 with the tfm_slih_test_service
 - Docs update:
   - Remove the out-of-date tfm_non-secure_interrupt_handling.rst
   - Remove IRQ test part in core_test_services_integration_guide.rst
     as core test does not participate in IRQ testing anymore.

Note:
  The new SLIH testing does not support Library Model.

Change-Id: I578d43ca7a363cdcb298226cc9ebad5f7b66fb48
Signed-off-by: Kevin Peng <kevin.peng@arm.com>