Restructure PIV/CAC setup spec for consistency

[aduth]

🎫 Ticket

Refactoring related to LG-14455

🛠 Summary of changes

Updates specs for Users::PivCacAuthenticationSetupController to fix inaccuracies and follow conventions:

• Specs described #index where the controller has no index action, fixed to #new
• Specs used describe to prescribe contextual scenarios, updated to context
• Specs were written context-first, rather than action-first

Easier to review with whitespace changes hidden: https://github.com/18F/identity-idp/pull/11355/files?w=1

Example before/after:

# Before:

describe 'when not signed in' do
  describe '#index' do
    it '...' do
      # ...
    end
  end
end

describe 'when signed out' do
  describe '#index' do
    it '...' do
      # ...
    end
  end
end

# After

describe '#new' do
  context 'when not signed in' do
    it '...' do
      # ...
    end
  end

  context 'when signed out' do
    it '...' do
      # ...
    end
  end
end

📜 Testing Plan

rspec spec/controllers/users/piv_cac_authentication_setup_controller_spec.rb

[kevinsmaster5]

Looks good! 👍