Make ReconciliationService logic more flexible for tests

The following discussion from !9261 should be addressed:

• @ebaque started a discussion: (+1 comment)

  This is not the first time that I'm seeing this amount being updated in this spec file. A while ago, I tried to mitigate the need to update by adding some "flexibility" to an assertion:

  # spec/services/reconciliations/order_api_spec.rb:72
  
        # The amount_without_tax returned from Zuora can vary a bit based on number of days remaining in subscription (which can differ slightly based on whether remaining
        # months have 30 or 31 days).
        expect(invoice.amount_without_tax).to be_within(amount_without_tax * 0.01).of(amount_without_tax)

  This time around, how about then adding some related logic in the ReconciliationService? Something like:

  # app/services/reconciliation_service.rb
  
  def raise_error(details)
    return if Rails.env.test? && diff_within_tolerance_for?(details)
  
    ...
  end
  
  def diff_within_tolerance_for?(details)
    (details[:quoted_invoice_amount] / details[:latest_preview_amount]).between?(0.99, 1.01)
  end

  With something like this, we won't have to update this amount in the spec file periodically. WDYT?