tests: arch: arm: Correctly report skipped tests as skipped

test_arm_user_interrupt and test_arm_null_pointer_exception currently
only print "Skipped" but reports as PASS in twister. This fixes that
misleading report.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>
diff --git a/tests/arch/arm/arm_interrupt/src/arm_interrupt.c b/tests/arch/arm/arm_interrupt/src/arm_interrupt.c
index 3891c9b..425816c 100644
--- a/tests/arch/arm/arm_interrupt/src/arm_interrupt.c
+++ b/tests/arch/arm/arm_interrupt/src/arm_interrupt.c
@@ -455,16 +455,17 @@
 #else
 ZTEST_USER(arm_interrupt, test_arm_user_interrupt)
 {
-	TC_PRINT("Skipped\n");
+	ztest_test_skip();
 }
 #endif /* CONFIG_USERSPACE */
 
-#if defined(CONFIG_CORTEX_M_NULL_POINTER_EXCEPTION)
 #pragma GCC push_options
 #pragma GCC optimize("O0")
 /* Avoid compiler optimizing null pointer de-referencing. */
 ZTEST(arm_interrupt, test_arm_null_pointer_exception)
 {
+	Z_TEST_SKIP_IFNDEF(CONFIG_CORTEX_M_NULL_POINTER_EXCEPTION);
+
 	int reason;
 
 	struct test_struct {
@@ -483,13 +484,6 @@
 		"expected_reason has not been reset (%d)\n", reason);
 }
 #pragma GCC pop_options
-#else
-ZTEST(arm_interrupt, test_arm_null_pointer_exception)
-{
-	TC_PRINT("Skipped\n");
-}
-
-#endif /* CONFIG_CORTEX_M_NULL_POINTER_EXCEPTION */
 
 /**
  * @}