Add backend test harness with mock SQLite

GameList.Tests/Support/StubHttpMessageHandler.cs

@@ -0,0 +1,35 @@
+using System.Net;
+using System.Net.Http.Headers;
+
+namespace GameList.Tests.Support;
+
+internal class StubHttpMessageHandler : HttpMessageHandler
+{
+    private Func<HttpRequestMessage, HttpResponseMessage> _responder;
+
+    public StubHttpMessageHandler()
+    {
+        _responder = DefaultResponder;
+    }
+
+    public void SetResponder(Func<HttpRequestMessage, HttpResponseMessage> responder)
+    {
+        _responder = responder ?? DefaultResponder;
+    }
+
+    private static HttpResponseMessage DefaultResponder(HttpRequestMessage _)
+    {
+        var response = new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new ByteArrayContent(Array.Empty<byte>())
+        };
+        response.Content.Headers.ContentType = new MediaTypeHeaderValue("image/png");
+        response.Content.Headers.ContentLength = 0;
+        return response;
+    }
+
+    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+    {
+        return Task.FromResult(_responder(request));
+    }
+}