selftests/nolibc: Use printf variable field widths and precisions

selftests/nolibc: Use printf variable field widths and precisions

Now that printf supports '*' for field widths and precisions
then can be used to simplify the test output.
 - aligning the "[OK]" strings.
 - reporting the expected sprintf() output when there is a mismatch.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260308113742.12649-18-david.laight.linux@gmail.com
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>