Add coverage for tested API operations

[skryukov]

This PR adds coverage for tested API operations:

# spec/rails_helper.rb

RSpec.configure do |config|
  # To enable coverage, pass `coverage: :report` option,
  # and to raise an error when an operation is not covered, pass `coverage: :strict` option:
  config.include Skooma::RSpec[Rails.root.join("docs", "openapi.yml"), coverage: :report], type: :request
end

$ bundle exec rspec
# ...
OpenAPI schema /openapi.yml coverage report: 110 / 194 operations (56.7%) covered.
Uncovered paths:
GET /api/uncovered 200
GET /api/partially_covered 403
# ...