Prefer `instance_double` in testing best practices

What does this MR do?

Instead of double we should use instance_double so the stubbed methods are verified for us.

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.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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