Remove superfluous printing from tests

Our test_runner now prints these messages automatically (see #816 and #817).

blog/content/second-edition/posts/11-allocator-designs/index.md

@@ -402,14 +402,12 @@ The main limitation of a bump allocator is that it can only reuse deallocated me
 
 #[test_case]
 fn many_boxes_long_lived() {
-    serial_print!("many_boxes_long_lived... ");
     let long_lived = Box::new(1); // new
     for i in 0..HEAP_SIZE {
         let x = Box::new(i);
         assert_eq!(*x, i);
     }
     assert_eq!(*long_lived, 1); // new
-    serial_println!("[ok]");
 }
 ```