string: Prepare to merge strscpy_kunit.c into string_kunit.c

string: Prepare to merge strscpy_kunit.c into string_kunit.c

In preparation for moving the strscpy_kunit.c tests into string_kunit.c,
rename "tc" to "strscpy_check" for better readability.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20240419140155.3028912-1-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>