lib/test_string.c: add strspn and strcspn tests

Before refactoring strspn() and strcspn(), add some simple test cases.

Link: https://lkml.kernel.org/r/20220328224119.3003834-1-linux@rasmusvillemoes.dk


Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>