There are already 2 other KUnit tests in `lib/` covering different groups of
string functions separately (lib/strscpy_kunit.c, lib/strcat_kunit.c), so
this patch just follows this pattern. I believe it makes sense: the tests
are separated to cover one specific group of string functions with a similar
purpose
We have handful of the string functions, are you going to have a file per
function? Isn't it way too many?
P.S>
Having those does not prove it's a correct approach. I would rather expect
somebody to incorporate those into string_kunit.c.