Move roundtrip tests to their own namespace.

This makes it easier to run the fast non-roundtrip tests.

ICSharpCode.Decompiler.Tests/Helpers/Tester.cs

@@ -528,7 +528,7 @@ namespace ICSharpCode.Decompiler.Tests.Helpers
 			{
 				CompilerResults results = new CompilerResults();
 				results.PathToAssembly = outputFileName;
-				string testBasePath = RoundtripAssembly.TestDir;
+				string testBasePath = Roundtrip.RoundtripAssembly.TestDir;
 				if (!Directory.Exists(testBasePath))
 				{
 					Assert.Ignore($"Compilation with mcs ignored: test directory '{testBasePath}' needs to be checked out separately." + Environment.NewLine +

ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs

@@ -33,7 +33,7 @@ using ICSharpCode.Decompiler.Tests.Helpers;
 
 using NUnit.Framework;
 
-namespace ICSharpCode.Decompiler.Tests
+namespace ICSharpCode.Decompiler.Tests.Roundtrip
 {
 	[TestFixture, Parallelizable(ParallelScope.All)]
 	public class RoundtripAssembly