http://stackoverflow.com/questions/25605130/testing-f-async-workflows-with-xunit-nets-task-support

gistfile1.fs

#r @"c:\prg\Fuchu\Fuchu\bin\Debug\Fuchu.dll" // https://www.nuget.org/packages/Fuchu/

open System
open Fuchu

// Write your tests as a list of string * Async<unit>
let tests = [
    "Check google response", async {
        use client = new System.Net.WebClient()
        let! a = client.AsyncDownloadString (Uri("http://www.google.com"))
        Assert.StringContains("google response", expectedSubString = "Google", actual = a)
    }

    "Check Bing response", async {
        use client = new System.Net.WebClient()
        let! a = client.AsyncDownloadString (Uri("http://www.bing.com"))
        Assert.StringContains("bing response", expectedSubString = "Microsoft", actual = a)
    }
]

// Then run them
tests |> Seq.map (fun (n,a) -> testCase n (fun () -> Async.RunSynchronously a)) |> TestList |> runParallel

Or try https://github.com/GreanTech/Exude if you insist on using xUnit.