Warn on context fixture helper

[cupakromer]

Fixtures are controlled by the underlying ActiveRecord::TestFixtures code. Which sets up fixtures before each example. While the database may have fixture data in it before this point, that data will be reset once the test fixture code runs.

When the fixtures message is sent in an example group, or global fixtures are defined, the Active Record test fixtures register the accessors in an anonymous module. That module is included in the example group's ancestors.

Which means the accessor helpers are available in any example group instance; thus they are available in before(:context) and after(:context). However, the internal fixture setup and cache has not been performed yet. Attempting to call a fixture helper prior to the fixtures being setup, raises an undefined method on NilClass error.

Related: #1201 and #1208

[fables-tales]

Reproduction case is put a file called spec/models/widgets_spec.rb in a rails app with a "Widget" model.

require 'rails_helper'

RSpec.describe Widget, type: :model do
  fixtures :widgets

  describe 'fixture loaded in a before context' do
    before :context do
      widgets :a
    end

    it 'works' do
    end
  end
end

[fables-tales]

@cupakromer I have a sketch of a fix for this. I'm going to clean it up a little and PR it tomorrow.