Do not `eager_load` by default in the test environment

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

.gitlab/ci/rails.gitlab-ci.yml

@@ -168,7 +168,6 @@ setup-test-env:
     - .rails:rules:code-backstage-qa
   stage: prepare
   variables:
-    GITLAB_TEST_EAGER_LOAD: "0"
     SETUP_DB: "false"
   script:
     - run_timed_command "scripts/setup-test-env"

.gitpod.yml

@@ -76,8 +76,6 @@ tasks:
         sleep 5
         printf "$(date) – GitLab is up (took ~%.1f minutes)\n" "$((10*$SECONDS/60))e-1" | tee -a /workspace/startup.log
         gp preview $(gp url 3000) || true
-        # Speed up backend tests
-        export GITLAB_TEST_EAGER_LOAD=false
       )
 
 ports:

config/environments/test.rb

@@ -49,7 +49,7 @@ Rails.application.configure do
   # Print deprecation notices to the stderr
   config.active_support.deprecation = :stderr
 
-  config.eager_load = Gitlab::Utils.to_boolean(ENV['GITLAB_TEST_EAGER_LOAD'], default: true)
+  config.eager_load = Gitlab::Utils.to_boolean(ENV['GITLAB_TEST_EAGER_LOAD'], default: ENV['CI'].present?)
 
   config.cache_store = :null_store
 

doc/development/testing_guide/best_practices.md

@@ -50,6 +50,20 @@ bundle exec guard
 
 When using spring and guard together, use `SPRING=1 bundle exec guard` instead to make use of spring.
 
+### Eager loading the application code
+
+By default, the application code:
+
+- Isn't eagerly loaded in the `test` environment.
+- Is eagerly loaded in CI/CD (when `ENV['CI'].present?`) to surface any potential loading issues.
+
+If you need to enable eager loading when executing tests,
+use the `GITLAB_TEST_EAGER_LOAD` environment variable:
+
+```shell
+GITLAB_TEST_EAGER_LOAD=1 bin/rspec spec/models/project_spec.rb
+```
+
 ### Ruby warnings
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/47767) in GitLab 13.7.