Add comprehensive backend test suite and helper access

GameList.Tests/MiddlewareTests.cs

@@ -0,0 +1,28 @@
+using System.Net;
+using GameList.Tests.Support;
+
+namespace GameList.Tests;
+
+public class MiddlewareTests
+{
+    [Fact]
+    public async Task Deleted_player_cookie_is_signed_out()
+    {
+        using var factory = new TestWebApplicationFactory();
+        var client = factory.CreateClientWithCookies();
+        await client.RegisterAsync("ghost");
+
+        var playerId = await client.GetProfileIdAsync();
+
+        await factory.WithDbContextAsync(async db =>
+        {
+            var player = await db.Players.FindAsync(playerId);
+            db.Players.Remove(player!);
+            await db.SaveChangesAsync();
+        });
+
+        var resp = await client.GetAsync("/api/state");
+        Assert.Equal(HttpStatusCode.Unauthorized, resp.StatusCode);
+        Assert.Contains(resp.Headers, h => h.Key.Equals("Set-Cookie", StringComparison.OrdinalIgnoreCase));
+    }
+}