Fix roundtrip tests.

ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs

@@ -150,6 +150,9 @@ namespace ICSharpCode.Decompiler.Tests
 						resolver.RemoveSearchDirectory(".");
 						var decompiler = new TestProjectDecompiler(inputDir);
 						decompiler.AssemblyResolver = resolver;
+						// Let's limit the roundtrip tests to C# 7.3 for now; because 8.0 is still in preview
+						// and the generated project doesn't build as-is.
+						decompiler.Settings = new DecompilerSettings(LanguageVersion.CSharp7_3);
 						// use a fixed GUID so that we can diff the output between different ILSpy runs without spurious changes
 						decompiler.ProjectGuid = Guid.Parse("{127C83E4-4587-4CF9-ADCA-799875F3DFE6}");
 						decompiler.DecompileProject(module, decompiledDir);