selftests/nolibc: test_fork: fix up duplicated print

running nolibc-test with glibc on x86_64 got such print issue:

    29 execve_root = -1 EACCES                                       [OK]
    30 fork30 fork = 0                                                      [OK]
    31 getdents64_root = 712                                         [OK]

The fork test case has three printf calls:

    (1) llen += printf("%d %s", test, #name);
    (2) llen += printf(" = %d %s ", expr, errorname(errno));
    (3) llen += pad_spc(llen, 64, "[FAIL]\n"); --> vfprintf()

In the following scene, the above issue happens:

    (a) The parent calls (1)
    (b) The parent calls fork()
    (c) The child runs and shares the print buffer of (1)
    (d) The child exits, flushs the print buffer and closes its own stdout/stderr
        * "30 fork" is printed at the first time.
    (e) The parent calls (2) and (3), with "\n" in (3), it flushs the whole buffer
        * "30 fork = 0 ..." is printed

Therefore, there are two "30 fork" in the stdout.

Between (a) and (b), if flush the stdout (and the sterr), the child in
stage (c) will not be able to 'see' the print buffer.
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/nolibc/nolibc-test.c

@@ -486,7 +486,13 @@ static int test_getpagesize(void)
 static int test_fork(void)
 {
 	int status;
-	pid_t pid = fork();
+	pid_t pid;
+
+	/* flush the printf buffer to avoid child flush it */
+	fflush(stdout);
+	fflush(stderr);
+
+	pid = fork();
 
 	switch (pid) {
 	case -1: