Consider using `gazelle_generation_test` in our gazelle plugin tests

Consider using https://github.com/bazelbuild/bazel-gazelle/blob/master/extend.md#gazelle_generation_test in our `gazelle` plugin tests

Start with the linked Bazel Gazelle documentation for `gazelle_generation_test`, then locate the repository's gazelle plugin tests. Done means the relevant plugin tests use the documented test helper and continue to validate generated output.

索引モデルが issue の本文から書いたものです。