[libcxx] [test] Fix size_t-to-int truncation warnings in syserr.hash.

After r289363, these tests were triggering MSVC x64 warning C4267
"conversion from 'size_t' to 'int', possible loss of data" by taking 0, 2, and 10
as std::size_t, then constructing error_code(int, const error_category&) or
error_condition(int, const error_category&) from that (N4618 19.5.3.2
[syserr.errcode.constructors]/3, 19.5.4.2 [syserr.errcondition.constructors]/3).

The fix is simple: take these ints as int, pass them to the int-taking
constructor, and perform a value-preserving static_cast<std::size_t>
when comparing them to `std::size_t result`.

Fixes D27691.

llvm-svn: 289512