Modify run_pipe_test.

Author: wyfcyx

user/src/bin/run_pipe_test.rs

@@ -8,7 +8,7 @@ use user_lib::{exec, fork, wait};
 
 #[no_mangle]
 pub fn main() -> i32 {
-    for i in 0..50 {
+    for i in 0..5 {
         if fork() == 0 {
             exec("pipe_large_test\0", &[core::ptr::null::<u8>()]);
         } else {