string: Add tests for strndup (BZ #30266)

Copy strncpy tests for strndup.  Covers some basic testcases with random
strings.  Remove tests that set the destination's bytes and checked the
resulting buffer's bytes.  Remove wide character test support since
wcsndup() doesn't exist.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>