make gtest helper CO_TEST_ work on windows
make gtest helper CO_TEST_ work on windows

Summary:
getAsyncTrace doesn't exist on windows, so to make CO_TEST_ work on windows
let's avoid using getAsyncTrace there.

Differential Revision: D39764988

fbshipit-source-id: 3e7bf1959af5ff2a5b38a85496b7bfdf909321c8

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly