Tests for issue 6822

(The implementation to fix this is in the following commit.)
diff --git a/csharp/generate_protos.sh b/csharp/generate_protos.sh
index 06daa4e..3829d05 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -41,6 +41,9 @@
     src/google/protobuf/wrappers.proto
 
 # Test protos
+# Note that this deliberately does *not* include old_extensions1.proto
+# and old_extensions2.proto, which are generated with an older version
+# of protoc.
 $PROTOC -Isrc -Icsharp/protos \
     --csharp_out=csharp/src/Google.Protobuf.Test/TestProtos \
     --descriptor_set_out=csharp/src/Google.Protobuf.Test/testprotos.pb \