chore(tests): scaffold Chip8.Tests project with test harness

tests/Chip8.Tests/Chip8.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <RootNamespace>Chip8.Tests</RootNamespace>
+    <AssemblyName>Chip8.Tests</AssemblyName>
+    <LangVersion>10</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.0" />
+    <PackageReference Include="xunit" Version="2.4.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Chip8.Core\Chip8.Core.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Include="goldens\*.txt" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+
+</Project>

tests/Chip8.Tests/TestHarness.cs

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+using System;
+using System.IO;
+using Chip8.Core;
+
+namespace Chip8.Tests;
+
+static class TestHarness
+{
+    static string repoRoot()
+    {
+        var dir = new DirectoryInfo(AppContext.BaseDirectory);
+        while (dir is not null && !Directory.Exists(Path.Combine(dir.FullName, "roms")))
+        {
+            dir = dir.Parent;
+        }
+
+        if (dir is null)
+        {
+            throw new DirectoryNotFoundException("Could not locate repo root with a 'roms' directory.");
+        }
+
+        return dir.FullName;
+    }
+
+    public static byte[] romBytes(string name) => File.ReadAllBytes(Path.Combine(repoRoot(), "roms", name));
+
+    public static string goldenText(string name) =>
+        File.ReadAllText(Path.Combine(AppContext.BaseDirectory, "goldens", name));
+
+    public static Machine withOpcode(ushort opcode, Quirks? quirks = null)
+    {
+        var machine = new Machine(quirks ?? Quirks.modern(), randomSeed: 1);
+        machine.memory[Machine.programStart] = (byte)(opcode >> 8);
+        machine.memory[Machine.programStart + 1] = (byte)(opcode & 0xFF);
+        return machine;
+    }
+
+    public static Machine withOpcode(ushort opcode, Platform platform, Quirks? quirks = null)
+    {
+        var machine = new Machine(quirks, randomSeed: 1, platform);
+        machine.memory[Machine.programStart] = (byte)(opcode >> 8);
+        machine.memory[Machine.programStart + 1] = (byte)(opcode & 0xFF);
+        return machine;
+    }
+
+    public static Machine withProgram(Platform platform, params ushort[] words)
+    {
+        var machine = new Machine(null, randomSeed: 1, platform);
+        for (int index = 0; index < words.Length; index++)
+        {
+            machine.memory[Machine.programStart + index * 2] = (byte)(words[index] >> 8);
+            machine.memory[Machine.programStart + index * 2 + 1] = (byte)(words[index] & 0xFF);
+        }
+        return machine;
+    }
+
+    public const int instructionsPerFrame = 11;
+
+    public static Machine runRom(string name, int cycles, Quirks? quirks = null)
+    {
+        var machine = new Machine(quirks ?? Quirks.modern(), randomSeed: 1);
+        machine.loadRom(romBytes(name));
+        for (int cycle = 0; cycle < cycles; cycle++)
+        {
+            machine.step();
+            if (cycle % instructionsPerFrame == instructionsPerFrame - 1)
+            {
+                machine.tickTimers();
+            }
+        }
+        return machine;
+    }
+
+    public static Machine runRomExtended(string name, int cycles, Platform platform, Quirks? quirks = null)
+    {
+        var machine = new Machine(quirks, randomSeed: 1, platform);
+        machine.loadRom(romBytes(name));
+        for (int cycle = 0; cycle < cycles && !machine.exited; cycle++)
+        {
+            machine.step();
+            if (cycle % instructionsPerFrame == instructionsPerFrame - 1)
+            {
+                machine.tickTimers();
+            }
+        }
+        return machine;
+    }
+}