Summary: Added an arc unit test that checks that .pb.go and .gen.go files are updated and added to the diff.

Test Plan:
ran arc diff many times with different cases
- .proto file updated but pb.go is not
- .proto file updated and pb.go file updated
etc

Reviewers: zasgar, kgandhi

Reviewed By: zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D60

GitOrigin-RevId: 8c81e69422129bd57d75b826213d69fa3a76f627