Run InterfaceTests against the legacy csc and mcs compilers too

Widening the compiler matrix answers the open review question on the
Issue3230 fixture guard empirically: a class naming its own nested
interface in its base list is a Roslyn-era relaxation. The legacy csc
rejects every such shape with CS0146 (circular base class dependency,
it never reaches the accessibility check), mcs 2.6.4 rejects them with
CS0122/CS0146, and mcs 5.23 accepts them all, rejects naming a base
class's protected nested interface with the same CS0122 as Roslyn, and
emits the same transitive InterfaceImpl metadata. The fixtures are
therefore gated to ROSLYN || MCS5, which exercises the base-list filter
on mcs-generated metadata as well. The pre-existing class C needs
MCS2-specific expected output because mcs 2.6.4 reorders interface-impl
rows and explicit implementations in metadata.

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -921,7 +921,7 @@ namespace ICSharpCode.Decompiler.Tests
 		}
 
 		[Test]
-		public async Task InterfaceTests([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
+		public async Task InterfaceTests([ValueSource(nameof(defaultOptionsWithMcs))] CompilerOptions cscOptions)
 		{
 			await RunForLibrary(cscOptions: cscOptions);
 		}

ICSharpCode.Decompiler.Tests/TestCases/Pretty/InterfaceTests.cs

@@ -130,7 +130,13 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 #endif
+		// mcs 2.6.4 emits interface-impl rows depth-first and explicit implementations
+		// ahead of ordinary members.
+#if MCS2 && EXPECTED_OUTPUT
+		public class C : IA, IA2, IB
+#else
 		public class C : IA2, IA, IB
+#endif
 		{
 			int IA.Property1 {
 				get {
@@ -157,14 +163,22 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				remove {
 				}
 			}
+#if MCS2 && EXPECTED_OUTPUT
+			void IA.Method()
+			{
+				throw new NotImplementedException();
+			}
+#endif
 			public int Finalize()
 			{
 				return 0;
 			}
+#if !(MCS2 && EXPECTED_OUTPUT)
 			void IA.Method()
 			{
 				throw new NotImplementedException();
 			}
+#endif
 		}
 
 		internal interface IInterfacesCannotDeclareDtors
@@ -172,7 +186,9 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			int Finalize();
 		}
 
-#if ROSLYN
+		// Naming your own nested interface in the base list is a Roslyn-era relaxation
+		// shared by mcs 5.23; the legacy csc rejects it with CS0146, mcs 2.6.4 with CS0122.
+#if ROSLYN || MCS5
 		private class Issue3230_F : Issue3230_F.IFoo
 		{
 			protected interface IFoo