Usually, developers run a single test locally.
Disabling eager loading by default in test environment should increase
productivity since it saves around 10 seconds:

With `eager_load` as `true`:
```bash
bogdanvlviv@lenovo:~/gitlab-development-kit/gitlab$ bin/rspec spec/models/users_statistics_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Test environment set up in 5.122837013 seconds
......

Finished in 8.51 seconds (files took 16.46 seconds to load)
6 examples, 0 failures
```

With `eager_load` as `false`:
```bash
bogdanvlviv@lenovo:~/gitlab-development-kit/gitlab$ bin/rspec spec/models/users_statistics_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Test environment set up in 5.013345478 seconds
......

Finished in 8.24 seconds (files took 7.18 seconds to load)
6 examples, 0 failures
```

Inspired by https://github.com/rails/rails/pull/43508.

It is important to eager load app before deploying it to catch any
issues with constants loading.
I see that we disable eager_loading on CI, https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30368/diffs#c1e8bf49d38a49639de4f3b2f3cef9ebdbe50dd4_16_26
For mentioned reason above I thik we should enable eager loading on CI.
@rymai What do you think about that?

I also found that we 'Speed up backend tests in Gitpod environment',
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68482.
/cc @splattael

Remove `GITLAB_TEST_EAGER_LOAD: "0"` for scripts/setup-test-env
It doesn't make any difference for this script.
See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72964#note_713013642

Document `GITLAB_TEST_EAGER_LOAD`.
Closes https://gitlab.com/gitlab-org/gitlab/-/issues/343751