Do not `eager_load` by default in the test environment

What does this MR do and why?

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:

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:

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, !30368 (diffs) For the mentioned reason above I think 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', !68482 (merged). /cc @splattael

Remove GITLAB_TEST_EAGER_LOAD: "0" for scripts/setup-test-env It doesn't make any difference for this script. See !72964 (comment 713013642)

Document GITLAB_TEST_EAGER_LOAD. Closes #343751 (closed)

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How to set up and validate locally

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Contributes to #255339 (closed)

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