Fix #272: use 4K stack for sem-speed-test tasks

On VxWorks, no minimum stack size is implemented,
and the implementation will create tasks with an
extremely small stack if "0" is passed in.

This causes the worker tasks to overrun the stack
on Vxworks, triggering undefined/inconsistent
behavior.

src/tests/sem-speed-test/sem-speed-test.c

@@ -150,10 +150,10 @@ void SemSetup(void)
    /*
    ** Create the tasks
    */
-   status = OS_TaskCreate( &task_1_id, "Task 1", task_1, NULL, 0, TASK_PRIORITY, 0);
+   status = OS_TaskCreate( &task_1_id, "Task 1", task_1, NULL, 4096, TASK_PRIORITY, 0);
    UtAssert_True(status == OS_SUCCESS, "Task 1 create Id=%u Rc=%d", (unsigned int)task_1_id, (int)status);
 
-   status = OS_TaskCreate( &task_2_id, "Task 2", task_2, NULL, 0, TASK_PRIORITY, 0);
+   status = OS_TaskCreate( &task_2_id, "Task 2", task_2, NULL, 4096, TASK_PRIORITY, 0);
    UtAssert_True(status == OS_SUCCESS, "Task 2 create Id=%u Rc=%d", (unsigned int)task_2_id, (int)status);
 
    /* A small delay just to allow the tasks