C# test changes to account for internal/external differences.

PiperOrigin-RevId: 508402030

csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs

@@ -397,6 +397,11 @@ namespace Google.Protobuf.Reflection
         [Test]
         public void DescriptorImportingExtensionsFromOldCodeGen()
         {
+            if (MethodOptions.Descriptor.FullName != "google.protobuf.MethodOptions")
+            {
+                Assert.Ignore("Embedded descriptor for OldExtensions expects google.protobuf reflection package.");
+            }
+
             // The extension collection includes a null extension. There's not a lot we can do about that
             // in itself, as the old generator didn't provide us the extension information.
             var extensions = TestProtos.OldGenerator.OldExtensions2Reflection.Descriptor.Extensions;