Handle backspace for k210/qemu platform && Add lots of user tests.

user/src/bin/sleep_simple.rs

@@ -0,0 +1,19 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{get_time, sleep};
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("into sleep test!");
+    let start = get_time();
+    println!("current time_msec = {}", start);
+    sleep(100);
+    let end = get_time();
+    println!("time_msec = {} after sleeping 100 ticks, delta = {}ms!", end, end - start);
+    println!("r_sleep passed!");
+    0
+}