[nrf fromlist] tests: kernel: sched: increase test stack for thread objects

On nrf52dk/nrf52832, test_slice_reset overflows its Ztest thread.
The test keeps three struct k_thread objects on that stack while
creating and scheduling worker threads.

Use a 2048-byte stack for kernel.scheduler.slice_perthread.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>

Upstream PR #: 119471

Author: jaz1-nordic

tests/kernel/sched/schedule_api/tests.yaml

@@ -21,6 +21,7 @@ tests:
     extra_configs:
       - CONFIG_TIMESLICING=y
       - CONFIG_TIMESLICE_PER_THREAD=y
+      - CONFIG_ZTEST_STACK_SIZE=2048
   kernel.scheduler.multiq:
     extra_args: CONF_FILE=prj_multiq.conf
     extra_configs: