scanf: convert self-test to KUnit

scanf: convert self-test to KUnit

Convert the scanf() self-test to a KUnit test.

In the interest of keeping the patch reasonably-sized this doesn't
refactor the tests into proper parameterized tests - it's all one big
test case.

Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250307-scanf-kunit-convert-v9-3-b98820fa39ff@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>