Summary:
some of our gazelle builds were breaking because we always forget to run `make gazelle` after updating our go code.
this arc unit test checks that Gazelle builds properly (often breaks when adding new proto go files), and checks that `make gazelle` was run recently. if the unit test runs `make gazelle` and there are modified BUILD.bazel files, that means that the user forgot to run `make gazelle` and check in the updated build files.

Test Plan: try it out in different situations

Reviewers: zasgar, #engineering, philkuz

Reviewed By: #engineering, philkuz

Differential Revision: https://phab.corp.pixielabs.ai/D795

GitOrigin-RevId: b0939fd9214132cd75692b265d4f5839384f8a60