Force eager-loading for some specs that require it

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In !72964 (merged), we disabled eager-loading by default when running tests locally.

An issue with is that some tests rely on eager loading. Here are some examples:

1. Tests that search ObjectSpace may not be accurate if all objects are not loaded:

   • https://gitlab.com/gitlab-org/gitlab/blob/f8e1fb75d5eac0876ffdd249f26943799fdd0be7/spec/lib/banzai/pipeline/plain_markdown_pipeline_spec.rb#L35-47
   • https://gitlab.com/gitlab-org/gitlab/blob/a7f2f559e62a52d8d1adee1a6730e706efd6d0df/spec/models/concerns/reactive_caching_spec.rb#L378-384

2. Calls to Class.descendants / Class.subclasses also have the same problem because it uses ObjectSpace:

   • https://gitlab.com/gitlab-org/gitlab/blob/679ebf13709aa8440e58baa05539ad72b6e12e9b/spec/db/schema_spec.rb#L304-306
   • https://gitlab.com/gitlab-org/gitlab/blob/8ba2a598bab0475bd6f2ab9ebcef4a1852b8d512/spec/support/before_all_adapter.rb#L5
   • https://gitlab.com/gitlab-org/gitlab/blob/452d554d81b0e310ef0e86eb8ea3a1c7bf6fbf90/spec/support/helpers/migrations_helpers.rb#L70 (this may be fine since the intent is to reset the cache for already loaded models)
   • https://gitlab.com/gitlab-org/gitlab/blob/b982790579e7ba60a78971f3b82813d1deac9ad6/ee/spec/models/concerns/elastic/note_spec.rb#L196-200
   • https://gitlab.com/gitlab-org/gitlab/blob/6a1017b403b5efe170c45dd0823a5509fad86cc8/ee/spec/models/concerns/elastic/snippet_spec.rb#L67-71