tests: arch: arm_interrupt: Fix stack overflows in .no_optimizations

With e337b7b, the test started to
fail with stack overflows in the no_optimizations scenario, so give
the ztest thread a little more space.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>

Author: anangl

tests/arch/arm/arm_interrupt/testcase.yaml

@@ -16,6 +16,7 @@ tests:
       - CONFIG_ZTEST_WARN_NO_OPTIMIZATIONS=n
       - CONFIG_IDLE_STACK_SIZE=512
       - CONFIG_MAIN_STACK_SIZE=2048
+      - CONFIG_ZTEST_STACK_SIZE=1280
   arch.arm.interrupt.extra_exception_info:
     filter: not CONFIG_TRUSTED_EXECUTION_NONSECURE
     extra_configs: