head: fix 'test_spams_newline' to check *against* newline spam

The comment was introduced in commit 8320b1e,
the test was introduced in commit c1f518e
claiming to be about "failing GNU head tests".

However, a simple check reveals no such difference:

```console
$ echo -n a | hd
00000000  61                                                |a|
00000001
$ echo -n a | head | hd  # GNU head
00000000  61                                                |a|
00000001
$ echo -n a | cargo run -- head | hd
00000000  61                                                |a|
00000001
$ echo -n a | busybox head | hd
00000000  61                                                |a|
00000001
$
```

Looking at the GNU tests directly, it seems that there is a similar, but different test.

tests/by-util/test_head.rs

@@ -99,10 +99,8 @@ fn test_verbose() {
 }
 
 #[test]
-#[ignore]
 fn test_spams_newline() {
-    //this test is does not mirror what GNU does
-    new_ucmd!().pipe_in("a").succeeds().stdout_is("a\n");
+    new_ucmd!().pipe_in("a").succeeds().stdout_is("a");
 }
 
 #[test]