Enable RSpec/VerifiedDoubles cop rule

This MR enables 👮 RSpec/VerifiedDoubles.

In https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#optimize-factory-usage we are already mentioning instance_double (added via gitlab-org/gitlab!32085 (merged) and a nice addition to gitlab-org/gitlab!74630 (merged)).

Quoting https://relishapp.com/rspec/rspec-mocks/docs/verifying-doubles/using-an-instance-double:

An instance_double is the most common type of verifying double. It takes a class name or object as its first argument, then verifies that any methods being stubbed would be present on an instance of that class. In addition, when it receives messages, it verifies that the provided arguments are supported by the method signature, both in terms of arity and allowed or required keyword arguments, if any. The same argument verification happens when you constrain the arguments using with.

In cases where instance_double cannot be added easily we can simply disable this cop inline.

See https://relishapp.com/rspec/rspec-mocks/v/3-10/docs/basics/test-doubles

We recommend you use verifying doubles whenever possible.

Current offenses for gitlab-org/gitlab

24285 files inspected, 2757 offenses detected

See gitlab-org/gitlab!75885 (89a9cc5b) for a sneak preview.

🤦 master?? 🤦

Note, I've accidentally committed on master 🤦 and reverted immediately.