Add test for load_balancing initializer

What does this MR do and why?

Add test for load_balancing initializer. While not a perfect test (because 3 things are stubbed out), it functions as an early warning system.

I considered a feature test but it turns out that Capybara is incompatible with puma where workers > 0 so this is the best option. As a follow-up, a new QA scenario should be created to test that GitLab setup with DB load balancing works with our E2E tests.

Related issue: #333184 (closed)

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How to set up and validate locally

How to reproduce the original problem

1. Setup DB load balancing in development. The simplest way will be to add load_balancing: poiting to the same host:. Here's how my config/database.yml looks like:

development:
  main:
    adapter: postgresql
    encoding: unicode
    database: gitlabhq_development
    host: /Users/tkuah/code/gdk-ee/postgresql
    port: 5432
    pool: 10
    prepared_statements: false
    variables:
      statement_timeout: 120s
    load_balancing:
      hosts:
        - /Users/tkuah/code/gdk-ee/postgresql
        - /Users/tkuah/code/gdk-ee/postgresql
  ci:
    adapter: postgresql
    encoding: unicode
    database: gitlabhq_development_ci
    host: /Users/tkuah/code/gdk-ee/postgresql
    port: 5432
    pool: 10
    prepared_statements: false
    variables:
      statement_timeout: 120s
    load_balancing:
      hosts:
        - /Users/tkuah/code/gdk-ee/postgresql
        - /Users/tkuah/code/gdk-ee/postgresql

2. In config/initializers/load_balancing.rb, comment out the inside of the on_worker_start block
3. gdk restart
4. Navigate to any page. You should observe an error similar to #332913 (closed)

Running this new test:

1. Restore any changes to config/initializers/load_balancing.rb
2. bundle exec rspec spec/initializers/load_balancing_spec.rb. This test should pass. Note this test sets up DB load balancing for itself.
3. In config/initializers/load_balancing.rb, comment out the inside of the on_worker_start block
4. Run bundle exec rspec spec/initializers/load_balancing_spec.rb, observe the error / failure

MR acceptance checklist

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