add test case in user space for print speed

user/lib/src/bin/13speed.rs

+#![no_std]
+#![no_main]
+
+#[macro_use]
+extern crate user_lib;
+
+use user_lib::syscall::get_time;
+
+const SENTENCE: &str = "The big brother is watching you!";
+// const SENTENCE: &str = "B";
+const TIMES: usize = 32;
+
+#[no_mangle]
+fn main() {
+    let start = get_time();
+    for _ in 0..TIMES {
+        println!("{}", SENTENCE);
+    }
+    let end = get_time();
+    println!("Gap after print {} times: {}", TIMES, end - start);
+}
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