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

user/src/bin/forktest.rs

@@ -0,0 +1,34 @@
+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::{fork, wait, exit};
+
+const MAX_CHILD: usize = 20;
+
+#[no_mangle]
+pub fn main() -> i32 {
+    for i in 0..MAX_CHILD {
+        let pid = fork();
+        if pid == 0 {
+            println!("I am child {}", i);
+            exit(0);
+        } else {
+            println!("forked child pid = {}", pid);
+        }
+        assert!(pid > 0);
+    }
+    let mut exit_code: i32 = 0;
+    for _ in 0..MAX_CHILD {
+        if wait(&mut exit_code) <= 0 {
+            panic!("wait stopped early");
+        }
+    }
+    if wait(&mut exit_code) > 0 {
+        panic!("wait got too many");
+    }
+    println!("forktest pass.");
+    0
+}