Add a simple BrokerMessageListener L0 test (just to get that ball rolling)

src/Test/L0/Listener/BrokerMessageListenerL0.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Listener;
+using GitHub.Runner.Listener.Configuration;
+using GitHub.Services.Common;
+using Moq;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Listener
+{
+    public sealed class BrokerMessageListenerL0
+    {
+        private readonly RunnerSettings _settings;
+        private readonly Mock<IConfigurationManager> _config;
+        private readonly Mock<IBrokerServer> _brokerServer;
+        private readonly Mock<ICredentialManager> _credMgr;
+
+        public BrokerMessageListenerL0()
+        {
+            _settings = new RunnerSettings { AgentId = 1, AgentName = "myagent", PoolId = 123, PoolName = "default", ServerUrlV2 = "http://myserver", WorkFolder = "_work" };
+            _config = new Mock<IConfigurationManager>();
+            _config.Setup(x => x.LoadSettings()).Returns(_settings);
+            _brokerServer = new Mock<IBrokerServer>();
+            _credMgr = new Mock<ICredentialManager>();
+            _credMgr.Setup(x => x.LoadCredentials()).Returns(new VssCredentials());
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Runner")]
+        public async void CreatesSession()
+        {
+            using TestHostContext tc = CreateTestContext();
+            using var tokenSource = new CancellationTokenSource();
+            var guid = new Guid();
+
+            // Arrange
+            _brokerServer
+                .Setup(
+                    x => x.ConnectAsync(
+                        new Uri(_settings.ServerUrlV2),
+                        It.Is<VssCredentials>(y => y != null)
+                    )
+                )
+                .Returns(
+                    Task.FromResult(
+                        new TaskAgentSession { SessionId = guid }
+                    )
+                );
+            BrokerMessageListener listener = new();
+            listener.Initialize(tc);
+
+            // Act
+            var result = await listener.CreateSessionAsync(tokenSource.Token);
+
+            // Assert
+            Assert.True(result);
+        }
+
+        private TestHostContext CreateTestContext([CallerMemberName] String testName = "")
+        {
+            TestHostContext tc = new(this, testName);
+            tc.SetSingleton<IConfigurationManager>(_config.Object);
+            tc.SetSingleton<IBrokerServer>(_brokerServer.Object);
+            tc.SetSingleton<ICredentialManager>(_credMgr.Object);
+            return tc;
+        }
+    }
+}