Retarget main test project to `net48` to match EmuHawk

I swear the test runner didn't use to pick up Mono, otherwise I wouldn't have gone for .NET Core, but clearly it does now

src/BizHawk.Tests/BizHawk.Tests.csproj

@@ -1,9 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<PropertyGroup>
-		<TargetFramework>net8.0</TargetFramework>
-	</PropertyGroup>
-	<PropertyGroup Condition=" '$(APPVEYOR)' != '' ">
-		<TargetFramework>netcoreapp3.1</TargetFramework>
+		<TargetFramework>net48</TargetFramework>
 	</PropertyGroup>
 	<Import Project="../TestProjects.props" />
 	<PropertyGroup>

src/BizHawk.Tests/Client.Common/Movie/ZwinderStateManagerTests.cs

@@ -8,6 +8,32 @@ using Microsoft.VisualStudio.TestTools.UnitTesting;
 
 namespace BizHawk.Tests.Client.Common.Movie
 {
+#if !(NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER)
+	internal static class Extensions
+	{
+		public static void NextBytes(this Random rng, Span<byte> buf)
+		{
+			var a = buf.ToArray();
+			rng.NextBytes(a);
+			a.CopyTo(buf);
+		}
+
+		public static int Read(this BinaryReader br, Span<byte> buf)
+		{
+			var a = buf.ToArray();
+			var result = br.Read(a, index: 0, count: a.Length);
+			a.CopyTo(buf);
+			return result;
+		}
+
+		public static void Write(this BinaryWriter bw, ReadOnlySpan<byte> buf)
+			=> bw.Write(buf.ToArray());
+
+		public static void Write(this Stream stream, ReadOnlySpan<byte> buf)
+			=> stream.Write(buf.ToArray(), offset: 0, count: buf.Length);
+	}
+#endif
+
 	[TestClass]
 	public class ZwinderStateManagerTests
 	{

src/BizHawk.Tests/Client.Common/config/CorePickerStabilityTests.cs

@@ -2,6 +2,7 @@
 using System.Linq;
 
 using BizHawk.Client.Common;
+using BizHawk.Common;
 using BizHawk.Emulation.Cores;
 
 using Microsoft.VisualStudio.TestTools.UnitTesting;
@@ -27,7 +28,9 @@ namespace BizHawk.Tests.Client.Common.config
 			{
 				Assert.IsTrue(
 					appliesTo.Any(multiCoreSystems.Contains),
-					$"core picker has submenu for {appliesTo[0]} ({string.Join('/', appliesTo)}), but {(appliesTo.Length == 1 ? "that system doesn't have" : "none of those systems have")} alternate cores");
+					appliesTo.Length is 1
+						? $"core picker has submenu for {appliesTo[0]}, but that system doesn't have alternate cores"
+						: $"core picker has submenu for {appliesTo[0]} ({string.Join("/", appliesTo)}), but none of those systems have alternate cores");
 			}
 		}
 

src/BizHawk.Tests/Client.Common/config/SerializationStabilityTests.cs

@@ -3,7 +3,7 @@ using System.Collections.Generic;
 using System.Reflection;
 
 using BizHawk.Client.Common;
-
+using BizHawk.Common;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
 using Newtonsoft.Json;