Remove dependency on Zuora when running test suite

### Problem
As it stands currently, you must have the following secrets set for the test suite to pass.
```
zuora_api_user: <%= ENV['ZUORA_API_USER'] %>
zuora_api_password: '<%= ENV['ZUORA_API_PASSWORD'] %>'
```

Without this in place, you get an error like the following:
```
An error occurred while loading rails_helper.
Failure/Error:
          product_rate_plans.map do |product_rate_plan|
            product_rate_plan_charge = find_rate_plan_charge(product_rate_plan)
            product_rate_plan_charge_tier = find_rate_plan_charge_tier(product_rate_plan_charge)

            Plan.new(product_rate_plan, product_rate_plan_charge, product_rate_plan_charge_tier)
          end

Savon::SOAP::Fault:
  (fns:INVALID_LOGIN) invalid username or password: select Id, CreatedById, CreatedDate, Description, EffectiveEndDate, EffectiveStartDate, Name, ProductId, UpdatedById, UpdatedDate from ProductRatePlan where Id = '2c92c0f85991509d01599350751d762c' or Id = '2c92c0f853077d1d01530af9137f61bd' or Id = '2c92c0f95615e0560156272129075c2f' or Id = '2c92c0f958aa45650158d6ace53b53c5' or Id = '2c92c0f858aa38af0158d67f5eea4ca4' or Id = '2c92c0f958aa455e0158d1dcfd8427e6' or Id = '2c92c0f858aa39000158d67af26d6791' or Id = '2c92c0f861789eda01618c6e122f6bb5' or Id = '2c92c0f86179f24301618ceb8b456743' or Id = '2c92c0f861789eda01618c76c25810cb' or Id = '2c92c0f96178a7a901618cedb627046d' or Id = '2c92c0f86716796f01671a8e683821f2' or Id = '2c92c0f95a24621b015a259a50307881' or Id = '2c92c0f85aac66b3015aaea7164444f9' or Id = '2c92c0f95aac74f3015aaea8d97a4b48' or Id = '2c92c0f9699eca030169a2f1f117045f' or Id = '2c92c0f8540e24ee015411d434cf36ed' or Id = '2c92c0f9540e2d8d015411d39ac367dd' or Id = '2c92c0f9540e2d91015411d3402459d0' or Id = '2c92c0f9555cf11c015573e9db8731d8' or Id = '2c92c0f85599d51a01559c8759d31f9c' or Id = '2c92c0f85581f7a9015582383be8722f' or Id = '2c92c0f95819b30d015841863b87137e'
# ./app/models/plans_finder.rb:19:in `eager_load'
# ./config/initializers/zuora.rb:84:in `<top (required)>'
# ./config/environment.rb:5:in `<top (required)>'
# ./spec/rails_helper.rb:2:in `require'
# ./spec/rails_helper.rb:2:in `<top (required)>'
# ------------------
# --- Caused by: ---
# Savon::SOAP::Fault:
#   (fns:INVALID_SESSION) invalid session
#   ./app/models/plans_finder.rb:19:in `eager_load'
```

This error happens because the rails_helper.rb loads rails, which loads the config/initializers. This eventually loads the zuora.rb initializer and [tries to `eager_load` Plans here](https://gitlab.com/gitlab-org/customers-gitlab-com/blob/894d8c5c1cde3ae771ec1c41008f2f2ecddd2b1d/config/initializers/zuora.rb#L84). This fails because the `ZUORA_API_USER` and `ZUORA_API_PASSWORD` environment variables are not set (or really the secrets file doesn't have `zuora_api_user` and `zuora_api_password` set in the test section).

More info can be found in #645.

### Proposal
We could look into stubbing the `PlansFinder.eager_load` or maybe disable that part of the initializer in the test environment.

### Result
It would great to remove this dependency for multiple reasons:
- Simplifies the process for getting your test suite running locally.
- A big step towards being able to run the test suite without being connected to the internet.

### Next steps (if any)

Story points: 3.0