tests: kernel: mem_slab: Fix test for no multithreading

Modified test to exit early when multithreading is disabled to
fix linking errors.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
diff --git a/tests/kernel/mem_slab/mslab_api/src/test_mslab_api.c b/tests/kernel/mem_slab/mslab_api/src/test_mslab_api.c
index 521c9d5..3417932 100644
--- a/tests/kernel/mem_slab/mslab_api/src/test_mslab_api.c
+++ b/tests/kernel/mem_slab/mslab_api/src/test_mslab_api.c
@@ -133,11 +133,17 @@
 
 K_SEM_DEFINE(SEM_HELPERDONE, 0, 1);
 K_SEM_DEFINE(SEM_REGRESSDONE, 0, 1);
+static K_THREAD_STACK_DEFINE(stack, STACKSIZE);
+static struct k_thread HELPER;
 
-static void helper_thread(void)
+static void helper_thread(void *p0, void *p1, void *p2)
 {
 	void *ptr[BLK_NUM];           /* Pointer to memory block */
 
+	ARG_UNUSED(p0);
+	ARG_UNUSED(p1);
+	ARG_UNUSED(p2);
+
 	(void)memset(ptr, 0, sizeof(ptr));
 
 	k_sem_take(&SEM_REGRESSDONE, K_FOREVER);
@@ -170,9 +176,6 @@
 
 }  /* helper thread */
 
-K_THREAD_DEFINE(HELPER, STACKSIZE, helper_thread, NULL, NULL, NULL,
-		7, 0, 0);
-
 /*test cases*/
 /**
  * @brief Initialize the memory slab using k_mem_slab_init()
@@ -289,9 +292,18 @@
  */
 void test_mslab_pending(void)
 {
+	if (!IS_ENABLED(CONFIG_MULTITHREADING)) {
+		ztest_test_skip();
+		return;
+	}
+
 	int ret_value;
 	void *b;                        /* Pointer to memory block */
 
+	(void)k_thread_create(&HELPER, stack, STACKSIZE,
+			helper_thread, NULL, NULL, NULL,
+			7, 0, K_NO_WAIT);
+
 	k_sem_give(&SEM_REGRESSDONE);   /* Allow helper thread to run */
 
 	k_sem_take(&SEM_HELPERDONE, K_FOREVER);		/* Wait for helper thread to finish */