string: Merge strcat KUnit tests into string_kunit.c

Move the strcat() tests into string_kunit.c. Remove the separate
Kconfig and Makefile rule.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20240419140155.3028912-4-keescook@chromium.org


Signed-off-by: Kees Cook <keescook@chromium.org>