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

user/src/bin/yield.rs

@@ -0,0 +1,17 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+use user_lib::{getpid, yield_};
+
+#[no_mangle]
+pub fn main() -> i32 {
+    println!("Hello, I am process {}.", getpid());
+    for i in 0..5 {
+        yield_();
+        println!("Back in process {}, iteration {}.", getpid(), i);
+    }
+    println!("yield pass.");
+    0
+}