Add test for Timestamp and fix up wrong feature

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/SetHttpContextTimestampMiddleware.cs

@@ -20,7 +20,7 @@ public SetHttpContextTimestampMiddleware(TimeProvider timeProvider, RequestDeleg
 
     public Task InvokeAsync(HttpContext context)
     {
-        context.Features.Set<TimestampFeature>(new(_timeProvider));
+        context.Features.Set<ITimestampFeature>(new TimestampFeature(_timeProvider));
 
         return _next(context);
     }

test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/HttpContextIntegrationTests.cs

@@ -101,6 +101,14 @@ public Task RewritePathViaCoreApis()
             Assert.Equal("1", adapter.Request.QueryString["q"]);
         });
 
+    [Fact]
+    public Task Timestamp()
+        => RunTest("/", context =>
+        {
+            var timestamp = context.RequestServices.GetRequiredService<TimeProvider>().GetLocalNow().DateTime;
+            Assert.Equal(timestamp, context.GetAdapter().Timestamp);
+        });
+
     private static async Task RunTest(string path, Action<HttpContextCore> run)
     {
         // Arrange
@@ -111,6 +119,7 @@ private static async Task RunTest(string path, Action<HttpContextCore> run)
                     .UseTestServer()
                     .ConfigureServices(services =>
                     {
+                        services.AddSingleton<TimeProvider>(new MockTimeProvider());
                         services.AddRouting();
                         services.AddSystemWebAdapters();
                     })
@@ -138,4 +147,11 @@ private static async Task RunTest(string path, Action<HttpContextCore> run)
             await host.StopAsync();
         }
     }
+
+    private sealed class MockTimeProvider : TimeProvider
+    {
+        private static readonly DateTimeOffset now = DateTimeOffset.Now;
+
+        public override DateTimeOffset GetUtcNow() => now;
+    }
 }