[libc][windows] fix strlcpy tests

Generally, size_t is an alias for unsigned long long. In the strlcpy
tests, the return value of strlcpy (a size_t) is compared to an unsigned
long. On Linux unsigned long and unsigned long long are both 64 bits,
but on windows unsigned long is 32 bits. Since the macros require
identical types for both sides, this caused a build failure on windows.
This patch changes the constants to be explicit size_t values.

Differential Revision: https://reviews.llvm.org/D125917