Add support for Assert.MultipleAsync

[manfred-brands]

This is mainly for detecting suppressing dereferencing possible null values.

[MaceWindu]

Looks like this is related:

[Test]
public async ValueTask TestAsync()
{
    await Assert.MultipleAsync(async () =>
    {
        // NUnit2045: add Assert.Multiple
        Assert.That(await Get1(), Is.Not.Null);
        Assert.That(await Get2(), Is.Not.Null);
    });
}

private static ValueTask<string?> Get1() => ValueTask.FromResult((string?)"");
private static ValueTask<string?> Get2() => ValueTask.FromResult((string?)"");