printf: break kunit into test cases

Move all tests into `printf_test_cases`. This gives us nicer output in
the event of a failure.

Combine `plain_format` and `plain_hash` into `hash_pointer` since
they're testing the same scenario.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250307-printf-kunit-convert-v6-2-4d85c361c241@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>

Author: tamird