Consider moving non-null-safe tests to their own file

[osa1]

Original comment by a reviewer in cl/460200489

In https://github.com/google/protobuf.dart/blob/7a832c248a0656ec47e6d927e0c6e8705c6f48b1/protoc_plugin/test/generated_message_test.dart we test behavior of the protobuf library when called from non-null-safe code.

Since protoc_plugin is null-safe, to be able to do that we override the language version with // @dart=2.11.

That disables null safety in the entire file. Ideally if we move the null-safety tests to another file then we don't need to run all the other tests in non-null-safe mode, and when we drop support for use from non-null-safe code we just delete the file.

[osa1]

Done in #791 (merged as cc0f287).