Mark concurrency-related tests as "expected to fail" on MacOS (#635)
Mark concurrency-related tests as "expected to fail" on MacOS (#635)

* Skip tests which fail on macos due to `spawn`

* use xfail instead of skip

if these tests ever start passing we want to know

Co-authored-by: Michael Penkov <22a5b664cdad6a9fa63954294c9cd5c30c9d59fd@gmail.com>
RaRe-Technologies/smart_open,RaRe-Technologies/smart_open