Make strategy used easier to detect in callbacks

[CodeMeister]

Problem this feature will solve

Like many, I use the same factory with multiple strategies, based on the individual test.

Critically, I want the same code executed on completion, regardless of the strategy used.

With :create also calling :build I need to determine in the :after_build callback if I should execute the code or not.

The strategy can be found with context.instance_variable_get(:@build_strategy).to_sym, but that required a degree of familiarity with the code that most users won't have.

Desired solution

A helper method for the context would simplify the whole process:

factory :user do
  after(:build)   { |user, context| run_this_code() if context.strategy.build?}
  after(:stub)    { |user, context| run_this_code() }
  after(:create)  { |user, context| run_this_code() }
end

Additional context

I'll do a pull request for this feature.