seq_buf: Introduce KUnit tests

Add KUnit tests for the seq_buf API to ensure its correctness and
prevent future regressions, covering the following functions:
- seq_buf_init()
- DECLARE_SEQ_BUF()
- seq_buf_clear()
- seq_buf_puts()
- seq_buf_putc()
- seq_buf_printf()
- seq_buf_get_buf()
- seq_buf_commit()

$ tools/testing/kunit/kunit.py run seq_buf
=================== seq_buf (9 subtests) ===================
[PASSED] seq_buf_init_test
[PASSED] seq_buf_declare_test
[PASSED] seq_buf_clear_test
[PASSED] seq_buf_puts_test
[PASSED] seq_buf_puts_overflow_test
[PASSED] seq_buf_putc_test
[PASSED] seq_buf_printf_test
[PASSED] seq_buf_printf_overflow_test
[PASSED] seq_buf_get_buf_commit_test
===================== [PASSED] seq_buf =====================

Link: https://lore.kernel.org/r/20250717085156.work.363-kees@kernel.org


Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <kees@kernel.org>