Upgrade xUnit to the latest version & warn about invalid ConfigureAwa…

[kiminuo]

…it value & fix test

The PR enables the https://xunit.net/xunit.analyzers/rules/xUnit1030 analyzer which warns about this

using System.Threading.Tasks;
using Xunit;

public class xUnit1030
{
    [Fact]
    public async Task TestMethod()
    {
        await Task.Delay(1).ConfigureAwait(false); // this is incorrect, either remove ConfigureAwait or use `true` value

        // ...code running on thread pool thread...
    }
}

That's important because it can break how xUnit runs tests and that in turn can make a WW test flaky.

[adamPetho]

cACK, tests are passing.

[yahiheb]

LGTM