Add boilerplate for unit testing GameSharkDecoder

should fail without the following commit

src/BizHawk.Tests/Client.Common/cheats/CheatDecoderTests.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+using BizHawk.Client.Common;
+using BizHawk.Client.Common.cheats;
+
+namespace BizHawk.Tests.Client.Common.cheats
+{
+	[TestClass]
+	public class CheatDecoderTests
+	{
+		[AttributeUsage(AttributeTargets.Method)]
+		private sealed class CheatcodeDataAttribute : Attribute, ITestDataSource
+		{
+			public bool GenerateNonsense { get; set; } = false;
+
+			public IEnumerable<object?[]> GetData(MethodInfo methodInfo)
+				=> GenerateNonsense ? NonsenseData : RealData;
+
+			public string GetDisplayName(MethodInfo methodInfo, object?[] data)
+				=> $"{methodInfo.Name}({string.Join(", ", data.Select(o => o is int i ? $"0x{i:X}" : o?.ToString() ?? "null"))})";
+		}
+
+		private const string ERROR_GBA_CODEBREAKER = "Codebreaker/GameShark SP/Xploder codes are not yet supported.";
+
+		private static readonly int? NO_COMPARE = null;
+
+		private static readonly IEnumerable<object?[]> NonsenseData = new[]
+		{
+			new[] { "GBA", "33003D0E0020", ERROR_GBA_CODEBREAKER },
+		};
+
+		private static readonly IEnumerable<object?[]> RealData = new[]
+		{
+			new object?[] { "GBA", "4012F5B7 3B7801A6", 0x00000006, 0xB7, NO_COMPARE, WatchSize.Byte },
+			new object?[] { "GBA", "686D7FC3 24B5B832", 0x00000032, 0x7FC3, NO_COMPARE, WatchSize.Word },
+		};
+
+		[DataTestMethod]
+		[CheatcodeData]
+		public void TestCheatcodeParsing(string systemID, string code, int address, int value, int? compare, WatchSize size)
+		{
+			var result = new GameSharkDecoder(null, systemID).Decode(code);
+			Assert.IsTrue(result.IsValid(out var valid), "failed to parse");
+			Assert.AreEqual(address, valid.Address, "wrong addr");
+			Assert.AreEqual(size, valid.Size, "wrong size");
+			Assert.AreEqual(
+				value,
+				valid.Size switch
+				{
+					WatchSize.Byte => valid.Value & 0xFF,
+					WatchSize.Word => valid.Value & 0xFFFF,
+					_ => valid.Value
+				},
+				"wrong value");
+			Assert.AreEqual(compare, valid.Compare, "wrong compare");
+		}
+
+		[DataTestMethod]
+		[CheatcodeData(GenerateNonsense = true)]
+		public void TestNonsenseParsing(string systemID, string code, string error)
+		{
+			var result = new GameSharkDecoder(null, systemID).Decode(code);
+			Assert.IsFalse(result.IsValid(out _), "parsed unexpectedly");
+			Assert.AreEqual(error, result.Error, "wrong error msg");
+		}
+	}
+}