Work around ABI differences due to LWG 2056 in tests

When testing against the system library, there is a relatively minor
ABI breakage that the std::future_errc values have been changed to
avoid using zero. Update the tests that rely on the values being
consistent.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@208840 91177308-0d34-0410-b5e6-96231b3b80d8