Quoting from pull request 43508 in the Rails repository [1]:
> When you are running test locally, most of the time you run only a
> subset, so it's better to load as little code as possible to have a
> faster time to first test result.
>
> But when you are on CI, it's usually much preferable to eager load the
> whole application because you will likely need all the code anyway,
> and even if the test suite is split across runners, it's preferable to
> load all the code to ensure any codefile that may have side effects is
> loaded.
>
> This also ensure that if some autoloaded constants are not properly
> tested on CI, at least they'll be loaded and obvious errors (e.g.
> SyntaxError) will be caught on CI rather than during deploy.
[1] https://github.com/rails/rails/commit/db0ee287eed
59f84deca1