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.

Written by the indexing model from the issue text.