[WIP] Alternative test base class created for really async specs (like xUnit async specs)

[IharBury]

The existing Chill specs allow specifying async givens and whens but run them synchronously via blocking waits. However some test frameworks (for example, xUnit) do support really async specs without any blocking. This PR adds an experimental base class for such specs. That should avoid problems mentioned in #48.

Also only one base class is used for all the scenarios (with and without subject, with and without result). That solves #44.

@Erwinvandervalk, @dennisdoomen, what do you think?

[IharBury]

See https://github.com/Erwinvandervalk/Chill/pull/50/files#diff-65ca5fe8309f5fa6dcf408c7e554e566 for the resulting syntax.

In short, with subject and result:

public void Test : AsyncTest<IGiven<Subject>, IWhen<Result>>
{
  public Test()
  {
    this.Given(async () =>
    {
      var subject = new Subject();
      await subject.PrepareAsync();
      return subject;
    });

    this.When(async subject =>
    {
      return await subject.DoIt();
    });
  }

  [Fact]
  public async void Then_it_rules()
  {
    Subject subject = await ExecuteGiven;
    Result result = await ExecuteWhen;
    result.Should().BeGreat();
  }
}

[IharBury]

I've modified the proposed architecture further to allow the subjects and the results to be not publicly visible (#49). See https://github.com/Erwinvandervalk/Chill/pull/50/files#diff-65ca5fe8309f5fa6dcf408c7e554e566 for the resulting syntax.

In short, with subject and result:

public void Test : TestBase
{
  private readonly AsyncTestContext<Subject, Result> test;

  public Test()
  {
    test = new AsyncTestContext<Subject, Result>(this);

    test.Given(async () =>
    {
      var subject = new Subject();
      await subject.PrepareAsync();
      return subject;
    });

    test.When(async subject =>
    {
      return await subject.DoIt();
    });
  }

  [Fact]
  public async Task Then_it_rules()
  {
    Subject subject = await test.ExecuteGiven();
    Result result = await test.ExecuteWhen();
    result.Should().BeGreat();
  }
}