Fix order-dependent Sidekiq config spec failures

What does this MR do and why?

This backports !140615 (merged) to the 16-7-stable-ee branch.

Previously the tests would fail if you ran:

bundle exec rspec spec/initializers/sidekiq_spec.rb spec/lib/gitlab/sidekiq_config_spec.rb

SidekiqConfig.load_cron_jobs! is memoized the first time it's run, so we need to clear this memoization in order to ensure the state doesn't leak from one test to another.