Warn on let_it_be duplications

[palkan]

Context

Although we suggest using let_it_be instead of let!, there is one important difference: you can override let! definition with the same or nested context, so only the latter one is called; let_it_be records could be overridden, but still created. For example:

context "A" do
  let!(:user) { create(:user, name: "a") } 
  let_it_be(:post) { create(:post, title: "A") }

  specify { expect(User.all.pluck(:name)).to eq ["a"] }
  specify { expect(Post.all.pluck(:title)).to eq ["A"] }

  context "B" do
    let!(:user) { create(:user, name: "b") }
    let_it_be(:post) { create(:post, title: "B") }

    specify { expect(User.all.pluck(:name)).to eq ["b"] }
    specify { expect(Post.all.pluck(:title)).to eq ["B"] } # fails, because there are two posts
  end
end

The situation becomes even more trickier when you have multiple let_it_be records and dependencies between them.

TODO

Add a configuration option to warn or raise an exception whenever a let_it_be definition is redefined:

TestProf::LetItBe.configure do |config|
  config.report_duplicates = :warn # Kernel.warn
  config.report_duplicates = :raise # Kernel.raise
end