Unit test example doesn't work

[NickLydon]

From https://github.com/dotnet/orleans-docs/blob/master/src/docs/implementation/testing.md/#L1

    [Fact]
    public async Task SaysHelloCorrectly()
    {
        var cluster = new TestCluster();
        cluster.Deploy();

        var hello = cluster.GrainFactory.GetGrain<IHelloGrain>(Guid.NewGuid());
        var greeting = await hello.SayHello();

        cluster.StopAllSilos();

        Assert.Equal("Hello, World", greeting);
    }

TestCluster doesn't have a parameterless constructor, and it's not obvious what's needed to get it going. Should its usage be completely replaced by TestClusterBuilder?

[RiskRunner0]

Hey Nick,

I was digging through the Orleans code, and I found some inspiration for this here, try this:

    [Fact]
    public async Task SaysHelloCorrectly()
    {
        var builder = new TestClusterBuilder();
        builder.Options.ServiceId = Guid.NewGuid().ToString();
        var cluster = builder.Build();
        cluster.Deploy();

        var hello = cluster.GrainFactory.GetGrain<IHelloGrain>(Guid.NewGuid());
        var greeting = await hello.SayHello();

        cluster.StopAllSilos();

        Assert.Equal("Hello, World", greeting);
    }

Looks like this is also part of the example later on, but yeah it should be up there with the SaysHelloCorrectly test too so you can actually compile and run.

[Expecho]

@QueenCitySmitty @ReubenBond Great, but now maybe we should remove the page https://dotnet.github.io/orleans/docs/implementation/testing.html since it contains exactly the same code but without those modifications. It seems to be a full duplicate.

[RiskRunner0]

@Expecho Good callout, I'll put out a PR in the docs

[RiskRunner0]

@Expecho here it is, take a quick look. Thanks!

[Expecho]

@QueenCitySmitty looks fine to me, thanks!

[Expecho]

@QueenCitySmitty found another issue with the docs, see #47