Set up automated test to compare protoc and protoparse output for src/google/protobuf/unittest.*proto?

[bufdev]

I can do this if you want.

What I'm thinking:

• Get all the unittest.*proto files from https://github.com/protocolbuffers/protobuf/tree/master/src/google/protobuf on an automated basis, either directly in a Golang test, or via a bash script.
• Build a FileDescriptorSet for each with protoc.
• Build a FileDescriptorSet for each with protoparse.
• Compare with proto.Equal.

We likely will have to exclude SourceCodeInfo for this type of automated test, but even without that, it's useful.

Also would recommend doing this for googleapis.

Both #307 and #308 were discovered by effectively doing this manually.

Note there's some prior art, I can adapt https://github.com/bufbuild/buf/blob/c75b6195beced011672dbffd4a0a11d004ccbca4/internal/buf/bufbuild/builder_test.go for this purpose (this does it all in Golang).