add test for clone of non-empty message (#9193)

* add test for clone

* use var

* reorder

csharp/src/Google.Protobuf.Test/UnknownFieldSetTest.cs

@@ -138,6 +138,29 @@ namespace Google.Protobuf
             Assert.AreEqual(message.ToByteArray(), otherEmptyMessage.ToByteArray());
         }
 
+        [Test]
+        public void TestClone_LengthDelimited()
+        {
+            var unknownVarintField = new UnknownField();
+            unknownVarintField.AddVarint(99);
+
+            var unknownLengthDelimitedField1 = new UnknownField();
+            unknownLengthDelimitedField1.AddLengthDelimited(ByteString.CopyFromUtf8("some data"));
+
+            var unknownLengthDelimitedField2 = new UnknownField();
+            unknownLengthDelimitedField2.AddLengthDelimited(ByteString.CopyFromUtf8("some more data"));
+
+            var destUnknownFieldSet = new UnknownFieldSet();
+            destUnknownFieldSet.AddOrReplaceField(997, unknownVarintField);
+            destUnknownFieldSet.AddOrReplaceField(999, unknownLengthDelimitedField1);
+            destUnknownFieldSet.AddOrReplaceField(999, unknownLengthDelimitedField2);
+
+            var clone = UnknownFieldSet.Clone(destUnknownFieldSet);
+
+            Assert.IsTrue(clone.HasField(997));
+            Assert.IsTrue(clone.HasField(999));
+        }
+
         [Test]
         [TestCaseSource(typeof(Data), "Messages")]
         public void TestDiscardUnknownFields(IMessage message)