Xunit: 3.0: Support for ValueTask

[Tornhoof]

ValueTask and ValueTask<T> are often allocation-free alternatives to Task and Task<T> respectively. They are used in many of the modern performance-oriented APIs in .NET Core 2.1+.
See https://github.com/dotnet/corefx/issues/27445 for more detail.

Test1 and Test2 in the example below (correctly) take ~1s to finish, Test3 and Test4 both complete after a few milliseconds.

Example:

public class UnitTest1
{
    [Fact]
    public Task Test1()
    {
        return Task.Delay(1000);
    }

    [Fact]
    public async Task Test2()
    {
        await new ValueTask(Task.Delay(1000));
    }

    [Fact]
    public ValueTask Test3()
    {
        return new ValueTask(Task.Delay(1000));
    }

    [Fact]
    public async ValueTask Test4()
    {
        await new ValueTask(Task.Delay(1000));
    }
}

[webcoyote]

This problem is actually a bit worse than mentioned, in that async tests that return ValueTask actually mask test failures. Both of these tests should fail, but only one does:

        [Fact]
        public async Task TaskTest () {
            await new ValueTask(Task.Delay(10));
            Assert.True(false);
        }

        [Fact]
        public async ValueTask ValueTaskTest () {
            await new ValueTask(Task.Delay(10));
            Assert.True(false);
        }
// <PackageReference Include="xunit" Version="2.4.1" />
// <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />

[bradwilson]

We don't support test methods that return values, so ValueTask<T> shouldn't be an issue here.