test_printf: Append strings more efficiently

Use scnprintf instead of snprintf + strlen.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20211019142621.2810043-5-willy@infradead.org