Add FileSystemEntry family classes

OpenTabletDriver.Tests/Fakes/FakeDirectory.cs

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+using System.Collections.Generic;
+
+namespace OpenTabletDriver.Tests.Fakes
+{
+    public class FakeDirectory : FakeFileSystemEntry
+    {
+        public List<FakeFileSystemEntry> Entries { get; } = new();
+    }
+}

OpenTabletDriver.Tests/Fakes/FakeFile.cs

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+using System;
+
+namespace OpenTabletDriver.Tests.Fakes
+{
+    public class FakeFile : FakeFileSystemEntry
+    {
+        public byte[] Data { get; init; } = Array.Empty<byte>();
+    }
+}

OpenTabletDriver.Tests/Fakes/FakeFileSystemEntry.cs

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+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using Xunit.Sdk;
+
+namespace OpenTabletDriver.Tests.Fakes
+{
+    public abstract class FakeFileSystemEntry
+    {
+        public string Name { get; init; } = null!;
+
+        public static async Task SetupFilesAsync(string directory, IEnumerable<FakeFileSystemEntry> entries)
+        {
+            foreach (var entry in entries)
+            {
+                if (entry is FakeDirectory dir)
+                {
+                    var dirPath = Path.Join(directory, dir.Name);
+                    Directory.CreateDirectory(dirPath);
+                    await SetupFilesAsync(dirPath, dir.Entries);
+                }
+                else if (entry is FakeFile file)
+                {
+                    var filePath = Path.Join(directory, file.Name);
+                    await File.WriteAllBytesAsync(filePath, file.Data);
+                }
+            }
+        }
+
+        public static async Task VerifyFilesAsync(string directory, IEnumerable<FakeFileSystemEntry> entries, bool strict = true)
+        {
+            var entriesCache = entries.ToArray();
+            var dirEntries = Directory.GetFileSystemEntries(directory);
+            var dirEntriesMap = dirEntries.ToDictionary(e => Path.GetFileName(e));
+
+            if (strict && (entriesCache.Length != dirEntries.Length))
+                throw new XunitException($"Expected {entriesCache.Length} file system entries, got {dirEntries.Length}");
+
+            foreach (var entry in entriesCache)
+            {
+                if (!dirEntriesMap.TryGetValue(entry.Name, out var path))
+                    throw new XunitException($"Expected file system entry '{entry.Name}' not found");
+
+                if (entry is FakeDirectory dir)
+                {
+                    if (!Directory.Exists(path))
+                        throw new DirectoryNotFoundException($"Directory '{path}' does not exist.");
+
+                    await VerifyFilesAsync(path, dir.Entries);
+                }
+                else if (entry is FakeFile file)
+                {
+                    if (!File.Exists(path))
+                        throw new FileNotFoundException($"File '{path}' does not exist.");
+
+                    var data = await File.ReadAllBytesAsync(path);
+                    if (!data.AsSpan().SequenceEqual(file.Data))
+                        throw new Exception($"File {path} does not match expected data");
+                }
+            }
+        }
+    }
+}