string: Prepare to merge strcat KUnit tests into string_kunit.c

string: Prepare to merge strcat KUnit tests into string_kunit.c

The test naming convention differs between string_kunit.c and
strcat_kunit.c. Move "test" to the beginning of the function name.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20240419140155.3028912-3-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>