Migrate from buildifier-diff to buildifier_test

The update of buildifier in https://github.com/tweag/gazelle_cabal/pull/58 introduced this deprecation warning (formatted for readability):
```
DEBUG: .../external/com_github_bazelbuild_buildtools/buildifier/internal/factory.bzl:17:10:
  DEPRECATION NOTICE: value 'diff' for attribute 'mode' will be removed in the future.
  Migrate '@//:buildifier-diff' to buildifier_test.
```