FSharp.Core.Unittests do not pass on Mono/OSX

[dsyme]

Running

./build.sh net40 test

on OSX after #4273 gave me a hang on FSHarp.Core.Unittests Async caching should work. It looks like it must be related to Async.Sleep. Note this is with a freshly compiled FSharp.Core.dll

This is odd, because we run these unit tests as part of CI on OSX in http://github.com/fsharp/fsharp. We should dig into what is different.

[cartermp]

Closing as this is no longer relevant