Tests for issue 6822

(The implementation to fix this is in the following commit.)

Makefile.am

diff --git a/Makefile.am b/Makefile.am
index 62e8a78..4550ce8 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -66,6 +66,8 @@
   csharp/keys/README.md                                                      \
   csharp/protos/README.md                                                    \
   csharp/protos/map_unittest_proto3.proto                                    \
+  csharp/protos/old_extensions1.proto                                        \
+  csharp/protos/old_extensions2.proto                                        \
   csharp/protos/unittest_custom_options_proto3.proto                         \
   csharp/protos/unittest_import_public_proto3.proto                          \
   csharp/protos/unittest_import_public.proto                                 \