Split kernel/user trap handler && Fix user tests.

os/src/task/mod.rs

@@ -2,7 +2,6 @@ mod context;
 mod switch;
 mod task;
 
-use crate::config::MAX_APP_NUM;
 use crate::loader::{get_num_app, get_app_data};
 use crate::trap::TrapContext;
 use core::cell::RefCell;

os/src/task/task.rs

@@ -71,7 +71,6 @@ impl TaskControlBlock {
 
 #[derive(Copy, Clone, PartialEq)]
 pub enum TaskStatus {
-    UnInit,
     Ready,
     Running,
     Exited,