Consider mocking out Flodesk::Client in specs

[mikej]

Follow up to #2321

Currently the Flodesk client is intentionally enabled when running in the test environment as a result of setting the Flodesk API to "test" in the initializer:

planner/config/initializers/flodesk.rb

Line 3 in 45fd9d1

key = Rails.env.test? ? 'test' : ENV['FLODESK_KEY']

This means all requests to Flodesk that are triggered during tests will be failing due to the invalid API key.

Requests will be triggered whenever we fabricate a student or a coach (as a result of a after_create callback on Subscription).

We can avoid the unnecessary requests, and potentially improve test performance by mocking out Flodesk::Client