Running RpcTests on all supported channels

[yallie]

Note: this pull request changes only the unit tests.

RpcTests currently test only default channel.
The idea is to subclass RpcTests to test Tcp and Websocket channels explicitly.
With this change it should be easier to test alternative channels such as System.Net.Websockets or QUIC.

Example:

public class RpcTests_Websockets : RpcTests
{
    protected override IServerChannel ServerChannel => new WebsocketServerChannel();

    protected override IClientChannel ClientChannel => new WebsocketClientChannel();

    public RpcTests_Websockets(ServerFixture fixture, ITestOutputHelper helper) : base(fixture, helper)
    {
    }
}